configure.ac: Fix call to AC_USE_SYSTEM_EXTENSIONS

The autoconf macro AC_USE_SYSTEM_EXTENSIONS does not take any arguments.

See:

    https://www.gnu.org/software/autoconf/manual/autoconf-2.69/html_node/Posix-Variants.html

    http://git.savannah.gnu.org/gitweb/?p=autoconf.git;a=blob;f=lib/autoconf/specific.m4;h=7d6be579294f0668554b2c598f6a972767c51f89;hb=771017a4336bcc38b99c84e345c94a8910b89996#l359

    https://lists.gnu.org/archive/html/autoconf/2014-02/msg00064.html

configure.ac

@@ -14,7 +14,7 @@ AM_SILENT_RULES([yes])
 AC_CONFIG_SRCDIR([src/knot])
 AC_CONFIG_HEADERS([src/config.h])
 AC_CONFIG_MACRO_DIR([m4])
-AC_USE_SYSTEM_EXTENSIONS([_GNU_SOURCE])
+AC_USE_SYSTEM_EXTENSIONS
 AC_CANONICAL_HOST
 
 AC_SUBST([KNOT_VERSION_MAJOR], knot_VERSION_MAJOR)