gnu: gobject-introspection: look for 'gcc' when no compiler seems available.

* gnu/packages/patches/gobject-introspection.patch: New file.

gnu/packages/patches/gobject-introspection-cc.patch

@@ -0,0 +1,14 @@
+--- a/giscanner/sourcescanner.py	2013-12-15 23:03:54.002937000 +0100
++++ b/giscanner/sourcescanner.py	2013-12-15 23:04:50.322937000 +0100
+@@ -277,6 +277,11 @@
+         defines = ['__GI_SCANNER__']
+         undefs = []
+         cpp_args = os.environ.get('CC', 'cc').split()  # support CC="ccache gcc"
++        if (cpp_args == ['cc'] and
++            not any(map(lambda x: os.access(os.path.join(x, 'cc'), os.X_OK),
++                        os.environ.get('PATH').split(':')))):
++            cpp_args = ['GUIX_GCC_PATH']
++
+         if 'cl' in cpp_args:
+             # The Microsoft compiler/preprocessor (cl) does not accept
+             # source input from stdin (the '-' flag), so we need