https://bugs.gentoo.org/713202
--- a/Make.common.in
+++ b/Make.common.in
@@ -36,7 +36,7 @@ AWK = @AWK@
 # Flags & libs
 # No way, to make make happy (except GNU), we cannot use := to append
 # something to these, so that's why there is a leading _
-XCFLAGS = @CFLAGS@
+XCFLAGS = @CFLAGS@ -D_GNU_SOURCE
 XCPPFLAGS = @CPPFLAGS@ -I.. -DBINDIR=\""$(bindir)/"\" -DLIBDIR=\""$(libdir)/"\" -DICONDIR=\""$(icondir)/"\" $(XINC)
 XLDFLAGS = @LDFLAGS@
 XDEFS = @DEFS@