define GNU_SOURCE so perl will be properly detected
--- ./configure.ac.orig
+++ ./configure.ac
@@ -67,6 +67,8 @@
 
 PKG_PROG_PKG_CONFIG
 
+AC_GNU_SOURCE
+
 AC_CACHE_CHECK([if bison is the parser generator],
 	[collectd_cv_prog_bison],
 	[AS_IF([$YACC --version 2>/dev/null | $EGREP -q '^bison '],