Remove explicit uses of _GNU_SOURCE.

Remove explicit uses of _GNU_SOURCE.

The best way to add these flags is to use the autoconf helper
AC_USE_SYSTEM_EXTENSIONS. This macro adds defines to config.h, so we
must include config.h (via para.h) before anything else. The two files
for which this matters are fixed by moving the include directive for
para.h to the top.

The AC_USE_SYSTEM_EXTENSIONS macro was introduced in Autoconf 2.60,
which was released ten years ago. The existing AC_PREREQ([2.61])
check in configure.ac makes sure the macro is defined.