autoconf: Restore _GNU_SOURCE for all Linux systems

This catches the linux-uclibc case and any others that were being set
prior to 98fcdf3f. Fixes bug 20345.

Signed-off-by: Dan Nicholson <dbn.lists@gmail.com>

configure.ac

@@ -87,7 +87,7 @@ dnl Compiler macros
 DEFINES=""
 AC_SUBST([DEFINES])
 case "$host_os" in
-*-gnu*)
+linux*|*-gnu*)
     DEFINES="$DEFINES -D_GNU_SOURCE -DPTHREADS"
     ;;
 solaris*)