Add #define _GNU_SOURCE to work around what seems to be Perl 5.8.0's

problem.  Per recent discussions about plperl failing to build on Linux.

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__)
 typedef unsigned char slock_t;