[WIP] Fix git gettext/libintl handling (builds on ubuntu) (#6859)

* Fix git on ubuntu, first cut

Spack needs to pass information about where the linker
can find `libintl`.  We're currently using `LDFLAGS` to do so.
The `LDFLAGS` info is pasted into the command line upstream
of the a file (`libgit.a`) that includes unresolved symbols that
need that library.  This fails on Ubuntu, although it seems to
work on CentOS (see #6841).

This change allows git to build on a Ubuntu 16.04.3 droplet.

TODO: test on other platforms...

* Add a bit of useful commentary

var/spack/repos/builtin/packages/git/package.py

@@ -165,13 +165,29 @@ class Git(AutotoolsPackage):
     depends_on('libtool',  type='build')
     depends_on('m4',       type='build')
 
+    # See the comment in setup_environment re EXTLIBS.
+    def patch(self):
+        filter_file(r'^EXTLIBS =$',
+                    '#EXTLIBS =',
+                    'Makefile')
+
     def setup_environment(self, spack_env, run_env):
-        # This is done to avoid failures when git is an external package.
+        # We use EXTLIBS rather than LDFLAGS so that git's Makefile
+        # inserts the information into the proper place in the link commands
+        # (alongside the # other libraries/paths that configure discovers).
+        # LDFLAGS is inserted *before* libgit.a, which requires libintl.
+        # EXTFLAGS is inserted *after* libgit.a.
+        # This depends on the patch method above, which keeps the Makefile
+        # from stepping on the value that we pass in via the environment.
+        #
+        # The test avoids failures when git is an external package.
         # In that case the node in the DAG gets truncated and git DOES NOT
         # have a gettext dependency.
         if 'gettext' in self.spec:
-            spack_env.append_flags('LDFLAGS', '-L{0} -lintl'.format(
+            spack_env.append_flags('EXTLIBS', '-L{0} -lintl'.format(
                 self.spec['gettext'].prefix.lib))
+            spack_env.append_flags('CFLAGS', '-I{0}'.format(
+                self.spec['gettext'].prefix.include))
 
     def configure_args(self):
         spec = self.spec