Do not define _POSIX_C_SOURCE

Because _GNU_SOURCE implies it, and causes it to be defined. And thus we
get problems when <config.h> gets included more than once. Which it will
do when we add the seqno test...

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

configure.ac

@@ -30,8 +30,6 @@ symver_win32_strerror=
 case $host_os in
  *linux* | *gnu* | *nacl*)
     AC_MSG_NOTICE([Applying feature macros for GNU build])
-    AC_DEFINE(_POSIX_C_SOURCE, 200112L, [_POSIX_C_SOURCE])
-    # For asprintf()
     AC_DEFINE(_GNU_SOURCE, 1, [_GNU_SOURCE])
     ;;
  *netbsd*)