bulid: use AC_USE_SYSTEM_EXTENSIONS instead of defining _GNU_SOURCE

Don't define _GNU_SOURCE on the command line. Instead, use
AC_USE_SYSTEM_EXTENSIONS which embeds it in "include/config.h".

Makefile.am

@@ -391,7 +391,6 @@ default_includes = \
 
 lib_cppflags = \
 	$(warn_cppflags) \
-	-D_GNU_SOURCE \
 	-DSYSCONFDIR=\"$(sysconfdir)/libnl\" \
 	$(default_includes) \
 	$(NULL)
@@ -709,7 +708,6 @@ EXTRA_src_lib_libnl_cli_3_la_DEPENDENCIES = \
 	libnl-cli-3.sym
 src_lib_libnl_cli_3_la_CPPFLAGS = \
 	$(warn_cppflags) \
-	-D_GNU_SOURCE \
 	-DPKGLIBDIR=\"$(pkglibdir)\" \
 	-DSYSCONFDIR=\"$(sysconfdir)\" \
 	$(default_includes) \
@@ -729,7 +727,6 @@ src_lib_libnl_cli_3_la_LIBADD = \
 
 src_cppflags = \
 	$(warn_cppflags) \
-	-D_GNU_SOURCE \
 	-DSYSCONFDIR=\"$(sysconfdir)/libnl\" \
 	$(default_includes) \
 	$(NULL)
@@ -909,7 +906,6 @@ src_nl_util_addr_LDADD =            $(src_ldadd)
 
 tests_cppflags = \
 	$(warn_cppflags) \
-	-D_GNU_SOURCE \
 	-DSYSCONFDIR=\"$(sysconfdir)/libnl\" \
 	$(default_includes) \
 	$(NULL)

configure.ac

@@ -61,6 +61,8 @@ AC_SUBST(LT_REVISION)
 LT_AGE=libnl_lt_age
 AC_SUBST(LT_AGE)
 
+AC_USE_SYSTEM_EXTENSIONS
+
 AC_PROG_CC
 AM_PROG_CC_C_O
 AC_PROG_CXX