configure: Remove duplicated use of AC_USE_SYSTEM_EXTENSIONS

Either the first or the second one is redundant so one should be
removed but beware, there's a bug introduced in automake 1.14
(fixed in 1.15) that would potentially cause trouble if you put it
before AM_INIT_AUTOMAKE.

More info about this automake bug here:
http://debbugs.gnu.org/cgi/bugreport.cgi?bug=15981

https://sourceforge.net/p/check/bugs/109

git-svn-id: svn+ssh://svn.code.sf.net/p/check/code/trunk@1216 64e312b2-a51f-0410-8e61-82d0ca0eb02a

AUTHORS

@@ -54,6 +54,7 @@ Contributors:
     Michał Dębski         (Use mkstemp() if available instead of tmpfile() or tempnam())
     Sebastian Dröge       (Kill running tests if SIGTERM or SIGINT are caught in test runner)
     Matt Clarkson         (Fix CMake checks using time.h for MinGW and MSVC)
+    Mario Sanchez Prada   (configure.ac cleanup)
 
 Anybody who has contributed code to Check or Check's build system is
 considered an author.  Send patches to this file to 

configure.ac

@@ -22,14 +22,12 @@ AC_CONFIG_LIBOBJ_DIR([lib])
 # place to put some extra build scripts installed
 AC_CONFIG_AUX_DIR([build-aux])
 
-# define things like _GNU_SOURCE appropriately
-AC_USE_SYSTEM_EXTENSIONS
-
 # really severe build strictness
 AM_INIT_AUTOMAKE([-Wall gnits 1.11.2])
 # Change to using into-in-builddir in the future:
 #AM_INIT_AUTOMAKE([info-in-builddir -Wall -Werror gnits 1.14])
 
+# define things like _GNU_SOURCE appropriately
 # From patch 2803433, request system extensions to generate 64-bit safe code
 AC_USE_SYSTEM_EXTENSIONS