-Wmaybe-uninitialized doesn't exist for NetBSD 6's gcc 4.5.3 and

breaks the build.  It does exist in NetBSD 7's gcc 4.8.4, but the
build is successful without it. So only apply the flag for gcc 5,
as it is necessary for 5.4.0.

Report from jnemeth@

sysutils/xenkernel45/Makefile

@@ -1,4 +1,4 @@
-# $NetBSD: Makefile,v 1.15 2016/05/21 16:07:01 prlw1 Exp $
+# $NetBSD: Makefile,v 1.16 2016/06/14 15:38:54 prlw1 Exp $
 
 VERSION=	4.5.3
 DISTNAME=	xen-${VERSION}
@@ -36,7 +36,7 @@ EXTRA_CFLAGS+=	-Qunused-arguments -no-integrated-as -Wno-error=format \
 		-Wno-error=unused-function -Wno-error=unused-const-variable \
 		-Wno-error=ignored-attributes -Wno-error=initializer-overrides
 .endif
-.if !empty(PKGSRC_COMPILER:Mgcc)
+.if !empty(CC_VERSION:Mgcc-5*)
 EXTRA_CFLAGS+=	-Wno-error=maybe-uninitialized
 .endif