Move -D_GNU_SOURCE hack from port header to template, so that

configure's tests see the same compilation environment as the code.
Per discussion with Stephan Szabo.

src/include/port/linux.h

-/* Force _GNU_SOURCE on; plperl is broken with Perl 5.8.0 otherwise */
-#ifndef _GNU_SOURCE
-#define _GNU_SOURCE 1
-#endif
-
-
 #if defined(__i386__) || defined(__x86_64__)
 typedef unsigned char slock_t;
 

src/template/linux

 CFLAGS=-O2
+# Force _GNU_SOURCE on; plperl is broken with Perl 5.8.0 otherwise
+CPPFLAGS="-D_GNU_SOURCE"
 
 SUPPORTS_THREADS=yes
 NEED_REENTRANT_FUNCS=yes	# verified glibc 2.1 2003-09-03