Hi,
quoting the Debian bug: http://bugs.debian.org/470796


When a C program that includes <glib/gutils.h> is built with
gcc -std=gnu99 or with gcc -std=c99, warnings are provoked by the GLib headers.
When gcc is invoked with -std=gnu89 (the default) or -std=c89, there are no
warnings.

Many projects use -Werror for compilation (at least for development versions),
and AC_PROG_CC_STDC selects -std=gnu99 by default, so this prevents compilation
of (for instance) development versions of Telepathy components.

A shell session reproducing the problem is shown below:

smcv@carbon% cp /usr/include/glib-2.0/glib/gutils.h mygutils.h
smcv@carbon% cat > tmp.c
#include "mygutils.h"
int main (void) { return 0; }
smcv@carbon% gcc -std=gnu89 `pkg-config --libs --cflags glib-2.0` tmp.c
smcv@carbon% gcc -std=gnu99 `pkg-config --libs --cflags glib-2.0` tmp.c
In file included from tmp.c:1:
mygutils.h:320: warning: C99 inline functions are not supported; using GNU89
mygutils.h:320: warning: to disable this warning use -fgnu89-inline or the gnu_inline function attribute
mygutils.h:334: warning: C99 inline functions are not supported; using GNU89
mygutils.h:347: warning: C99 inline functions are not supported; using GNU89
mygutils.h:366: warning: C99 inline functions are not supported; using GNU89
mygutils.h:374: warning: C99 inline functions are not supported; using GNU89
mygutils.h:391: warning: C99 inline functions are not supported; using GNU89
mygutils.h:400: warning: C99 inline functions are not supported; using GNU89

I suggest patching gutils.h like this:

--- gutils.h.orig       2008-03-13 18:14:53.000000000 +0000
+++ mygutils.h  2008-03-13 18:15:19.000000000 +0000
@@ -97,7 +97,7 @@
 #  define G_INLINE_FUNC
 #  undef  G_CAN_INLINE
 #elif defined (__GNUC__) 
-#  ifdef __GNUC_STDC_INLINE__
+#  if defined (__GNUC_STDC_INLINE__) || defined (__GNUC_GNU_INLINE__)
 #   define G_INLINE_FUNC extern inline __attribute__ ((__gnu_inline__))
 #  else
 #   define G_INLINE_FUNC extern inline

info gcc says:

   [the gnu_inline attribute] is available if either of the preprocessor
   macros `__GNUC_GNU_INLINE__' or `__GNUC_STDC_INLINE__' are defined

so I believe this is a correct patch. info gcc also says:

    In versions prior to 4.3, the only effect of explicitly including it is
    to disable warnings about using inline functions in C99 mode.