[giggle] configure: Use autoconf AC_GNU_SOURCE macro

[Javier Jardón <jjardon src gnome org>]

commit ad56b6abe77a401eda76266a86fcb7fc48675ea5
Author: Javier Jardón <jjardon gnome org>
Date:   Mon Jul 29 13:07:43 2013 +0100

    configure: Use autoconf AC_GNU_SOURCE macro

 configure.ac |    3 +--
 1 files changed, 1 insertions(+), 2 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 3e47b48..d56eef4 100644
--- a/configure.ac
+++ b/configure.ac
@@ -21,6 +21,7 @@ AM_SILENT_RULES([yes])
 AC_PROG_CC
 AC_PROG_CC_C_O
 AC_PROG_CC_C99
+AC_GNU_SOURCE
 gl_EARLY
 
 # For gnulib.
@@ -123,8 +124,6 @@ dnl  ----------------------
 dnl | library requirements |-----------------------------------
 dnl  ----------------------
 
-AC_DEFINE(_GNU_SOURCE, 1, [Enable GNU Extensions])
-
 dnl Make sure that strptime can be used (read: has gnu extensions)
 AC_MSG_CHECKING([for GNU extensions of strptime()])
 AC_RUN_IFELSE(