Get --enable-gcc-warnings working again.

Get --enable-gcc-warnings working again.

The recent changes to configure.ac removed the transliteration of
-I to -isystem in CFLAGS, which is needed for --enable-gcc-warnings.
Bring this back while keeping the spirit of the recent changes.
* configure.ac (edit_cflags): Restore this shell var, and put it
at the top level, where it'll be useful when emacs-24 is next merged.
(EMACS_CHECK_MODULES): New macro.  All uses of PKG_CHECK_MODULES
changed to use it.