Rework clock_gettime() test

The test for clock_gettime() in configure.in doesn't work properly
when a previous package has loaded the shared configuration cache with
informations about the availability of clock_gettime. A package such
as ctorrent does so, which means that compiling ctorrent *then*
libglib2 currently fails.

According to people on the Autoconf mailing list, the libglib2 test is
likely the one that needs to be fixed. The problem is that the
AC_CHECK_FUNCS() test assumes that if it finds clock_gettime() it
means that there's no need to add any -lrt flag to the
build. Unfortunately, due to the shared configuration cache, this test
is already done with -lrt, so the test succeeds, and libglib2 does not
know that it needs to add -lrt to G_THREAD_LIBS and
G_THREAD_LIBS_FOR_GTHREAD.

So instead, we remplace the test with an AC_SEARCH_LIBS() test,
followed by a test on the result of this AC_SEARCH_LIBS() test to add
the necessary -lrt to G_THREAD_LIBS and
G_THREAD_LIBS_FOR_GTHREAD. Therefore, even if the result for the
AC_SEARCH_LIBS() test is cached due to the prior execution ctorrent
./configure script, libglib2 ./configure will properly add -lrt to the
appropriate variables.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: glib-2.24.1/configure.in
===================================================================
--- glib-2.24.1.orig/configure.in	2010-08-11 19:29:20.530916023 +0200
+++ glib-2.24.1/configure.in	2010-08-11 19:46:41.308866269 +0200
@@ -2392,13 +2392,14 @@
 	LIBS="$glib_save_LIBS"
 fi
 
-AC_CHECK_FUNCS(clock_gettime, [], [
-  AC_CHECK_LIB(rt, clock_gettime, [
-    AC_DEFINE(HAVE_CLOCK_GETTIME, 1)
+AC_SEARCH_LIBS([clock_gettime], [rt], [
+    AC_DEFINE(HAVE_CLOCK_GETTIME, 1,[Have clock_gettime])
+])
+
+if test "$ac_cv_search_clock_gettime" = "-lrt"; then
     G_THREAD_LIBS="$G_THREAD_LIBS -lrt"
     G_THREAD_LIBS_FOR_GTHREAD="$G_THREAD_LIBS_FOR_GTHREAD -lrt"
-  ])
-])
+fi
 
 AC_CACHE_CHECK(for monotonic clocks,
     glib_cv_monotonic_clock,AC_COMPILE_IFELSE([AC_LANG_SOURCE([[