xerces-c: add variant for transcoder, rework config flags. (#9393)

* xerces-c: Add variant for choice of transcoder (gnuiconv, iconv, icu,
  macos, windows).  It's important to specify a --enable-transcoder
  option on the configure line or else xerces may make a different
  choice when multiple transcoders are available.
* Pass the compile flags to configure.  For cflags and cxxflags, this is
  necessary to respect the value from the spack install line.
  Otherwise, xerces (and any autotools package) will choose a default
  value that overrides the spack compiler wrapper.
* Add xerces version 3.2.2.
* icu4c needs the --enable-rpath configure option on Darwin

var/spack/repos/builtin/packages/icu4c/package.py

@@ -46,4 +46,11 @@ def url_for_version(self, version):
         return url.format(version.dotted, version.underscored)
 
     def configure_args(self):
-        return ['--enable-rpath']
+        args = []
+
+        # The --enable-rpath option is only needed on MacOS, and it
+        # breaks the build for xerces-c on Linux.
+        if 'platform=darwin' in self.spec:
+            args.append('--enable-rpath')
+
+        return args