AC_USE_SYSTEM_EXTENSIONS needed for Solaris

https://bugs.gentoo.org/show_bug.cgi?id=430090

--- a/configure.in
+++ b/configure.in
@@ -22,6 +22,7 @@
 fi
 
 AC_PROG_CC
+AC_USE_SYSTEM_EXTENSIONS
 AC_ISC_POSIX
 AC_HEADER_STDC