Back out the previous change. Now go back to hardcoding compiler to gcc.

Using CC fails when building gobject-introspection because CC is defined to
be $(SHELL) scripts/shave cc '$(SHAVE_SAVED_CC)'.  A more sophisticated fix
will be needed later.

giscanner/sourcescanner.py

@@ -250,7 +250,7 @@ def _parse(self, filenames):
 
         defines = ['__GI_SCANNER__']
         undefs = []
-        cpp_args = [os.environ.get('CC', 'gcc'), '-E', '-C', '-I.', '-']
+        cpp_args = ['gcc', '-E', '-C', '-I.', '-']
 
         cpp_args += self._cpp_options
         proc = subprocess.Popen(cpp_args,