Improve GCC warning and error triggers and also improve check of GCC version.

Configure snippets shamelessly stolen from http://www.libusb.org/browser/configure.ac

configure

@@ -746,6 +746,7 @@ ac_includes_default="\
 ac_subst_vars='am__EXEEXT_FALSE
 am__EXEEXT_TRUE
 LTLIBOBJS
+AM_CFLAGS
 CONFIG_DEFAULT_COMPLETE_FALSE
 CONFIG_DEFAULT_COMPLETE_TRUE
 LIBOBJS
@@ -4554,13 +4555,13 @@ if test "${lt_cv_nm_interface+set}" = set; then :
 else
   lt_cv_nm_interface="BSD nm"
   echo "int some_variable = 0;" > conftest.$ac_ext
-  (eval echo "\"\$as_me:4557: $ac_compile\"" >&5)
+  (eval echo "\"\$as_me:4558: $ac_compile\"" >&5)
   (eval "$ac_compile" 2>conftest.err)
   cat conftest.err >&5
-  (eval echo "\"\$as_me:4560: $NM \\\"conftest.$ac_objext\\\"\"" >&5)
+  (eval echo "\"\$as_me:4561: $NM \\\"conftest.$ac_objext\\\"\"" >&5)
   (eval "$NM \"conftest.$ac_objext\"" 2>conftest.err > conftest.out)
   cat conftest.err >&5
-  (eval echo "\"\$as_me:4563: output\"" >&5)
+  (eval echo "\"\$as_me:4564: output\"" >&5)
   cat conftest.out >&5
   if $GREP 'External.*some_variable' conftest.out > /dev/null; then
     lt_cv_nm_interface="MS dumpbin"
@@ -5766,7 +5767,7 @@ ia64-*-hpux*)
   ;;
 *-*-irix6*)
   # Find out which ABI we are using.
-  echo '#line 5769 "configure"' > conftest.$ac_ext
+  echo '#line 5770 "configure"' > conftest.$ac_ext
   if { { eval echo "\"\$as_me\":${as_lineno-$LINENO}: \"$ac_compile\""; } >&5
   (eval $ac_compile) 2>&5
   ac_status=$?
@@ -7296,11 +7297,11 @@ else
    -e 's:.*FLAGS}\{0,1\} :&$lt_compiler_flag :; t' \
    -e 's: [^ ]*conftest\.: $lt_compiler_flag&:; t' \
    -e 's:$: $lt_compiler_flag:'`
-   (eval echo "\"\$as_me:7299: $lt_compile\"" >&5)
+   (eval echo "\"\$as_me:7300: $lt_compile\"" >&5)
    (eval "$lt_compile" 2>conftest.err)
    ac_status=$?
    cat conftest.err >&5
-   echo "$as_me:7303: \$? = $ac_status" >&5
+   echo "$as_me:7304: \$? = $ac_status" >&5
    if (exit $ac_status) && test -s "$ac_outfile"; then
      # The compiler can only warn and ignore the option if not recognized
      # So say no if there are warnings other than the usual output.
@@ -7635,11 +7636,11 @@ else
    -e 's:.*FLAGS}\{0,1\} :&$lt_compiler_flag :; t' \
    -e 's: [^ ]*conftest\.: $lt_compiler_flag&:; t' \
    -e 's:$: $lt_compiler_flag:'`
-   (eval echo "\"\$as_me:7638: $lt_compile\"" >&5)
+   (eval echo "\"\$as_me:7639: $lt_compile\"" >&5)
    (eval "$lt_compile" 2>conftest.err)
    ac_status=$?
    cat conftest.err >&5
-   echo "$as_me:7642: \$? = $ac_status" >&5
+   echo "$as_me:7643: \$? = $ac_status" >&5
    if (exit $ac_status) && test -s "$ac_outfile"; then
      # The compiler can only warn and ignore the option if not recognized
      # So say no if there are warnings other than the usual output.
@@ -7740,11 +7741,11 @@ else
    -e 's:.*FLAGS}\{0,1\} :&$lt_compiler_flag :; t' \
    -e 's: [^ ]*conftest\.: $lt_compiler_flag&:; t' \
    -e 's:$: $lt_compiler_flag:'`
-   (eval echo "\"\$as_me:7743: $lt_compile\"" >&5)
+   (eval echo "\"\$as_me:7744: $lt_compile\"" >&5)
    (eval "$lt_compile" 2>out/conftest.err)
    ac_status=$?
    cat out/conftest.err >&5
-   echo "$as_me:7747: \$? = $ac_status" >&5
+   echo "$as_me:7748: \$? = $ac_status" >&5
    if (exit $ac_status) && test -s out/conftest2.$ac_objext
    then
      # The compiler can only warn and ignore the option if not recognized
@@ -7795,11 +7796,11 @@ else
    -e 's:.*FLAGS}\{0,1\} :&$lt_compiler_flag :; t' \
    -e 's: [^ ]*conftest\.: $lt_compiler_flag&:; t' \
    -e 's:$: $lt_compiler_flag:'`
-   (eval echo "\"\$as_me:7798: $lt_compile\"" >&5)
+   (eval echo "\"\$as_me:7799: $lt_compile\"" >&5)
    (eval "$lt_compile" 2>out/conftest.err)
    ac_status=$?
    cat out/conftest.err >&5
-   echo "$as_me:7802: \$? = $ac_status" >&5
+   echo "$as_me:7803: \$? = $ac_status" >&5
    if (exit $ac_status) && test -s out/conftest2.$ac_objext
    then
      # The compiler can only warn and ignore the option if not recognized
@@ -10179,7 +10180,7 @@ else
   lt_dlunknown=0; lt_dlno_uscore=1; lt_dlneed_uscore=2
   lt_status=$lt_dlunknown
   cat > conftest.$ac_ext <<_LT_EOF
-#line 10182 "configure"
+#line 10183 "configure"
 #include "confdefs.h"
 
 #if HAVE_DLFCN_H
@@ -10275,7 +10276,7 @@ else
   lt_dlunknown=0; lt_dlno_uscore=1; lt_dlneed_uscore=2
   lt_status=$lt_dlunknown
   cat > conftest.$ac_ext <<_LT_EOF
-#line 10278 "configure"
+#line 10279 "configure"
 #include "confdefs.h"
 
 #if HAVE_DLFCN_H
@@ -11104,6 +11105,81 @@ $as_echo "#define CONFIG_ANNOYING_NOISE /**/" >>confdefs.h
 fi
 
 
+# Restore gnu89 inline semantics on gcc 4.3 and newer
+saved_cflags="$CFLAGS"
+CFLAGS="$CFLAGS -fgnu89-inline"
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  inline_cflags="-fgnu89-inline"
+else
+  inline_cflags=""
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+CFLAGS="$saved_cflags"
+
+# check for -fvisibility=hidden compiler support (GCC >= 3.4)
+saved_cflags="$CFLAGS"
+CFLAGS="$CFLAGS -fvisibility=hidden"
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  VISIBILITY_CFLAGS="-fvisibility=hidden"
+
+$as_echo "#define API_EXPORTED __attribute__((visibility(\"default\")))" >>confdefs.h
+
+else
+   VISIBILITY_CFLAGS=""
+
+$as_echo "#define API_EXPORTED /**/" >>confdefs.h
+
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+CFLAGS="$saved_cflags"
+
+# check for -Wno-pointer-sign compiler support (GCC >= 4)
+saved_cflags="$CFLAGS"
+CFLAGS="$CFLAGS -Wno-pointer-sign"
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  nopointersign_cflags="-Wno-pointer-sign"
+else
+  nopointersign_cflags=""
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+CFLAGS="$saved_cflags"
+
+AM_CFLAGS="-std=gnu99 $inline_cflags -W -Wall -Wundef -Wunused -Wstrict-prototypes -Werror-implicit-function-declaration $nopointersign_cflags -Wshadow"
+
+
 ac_config_files="$ac_config_files Makefile src/Makefile include/Makefile man/Makefile examples/Makefile"
 
 cat >confcache <<\_ACEOF