Fix UPREFIX for Gentoo Prefix installations

Make sure UPREFIX in our make files gets set correctly when
MKPREFIX=yes. In this situation, UPREFIX should be ${PREFIX}/usr.

Reported-by: <heroxdb@gentoo.org>
X-Gentoo-Bug: 415899
X-Gentoo-Bug-URL: http://bugs.gentoo.org/show_bug.cgi?id=415899

mk/sys.mk

@@ -11,9 +11,14 @@ SED?=			sed
 SH=			/bin/sh
 
 PREFIX?=
-_UPREFIX_SH=		case "${PREFIX}" in "") echo /usr;; *) echo "${PREFIX}";; esac
-_UPREFIX:=		$(shell ${_UPREFIX_SH})
-UPREFIX=		${_UPREFIX}
+ifeq (${PREFIX},)
+UPREFIX= /usr
+else
+UPREFIX= ${PREFIX}
+ifeq (${MKPREFIX},yes)
+UPREFIX= ${PREFIX}/usr
+endif
+endif
 LOCAL_PREFIX=		/usr/local
 
 PICFLAG?=		-fPIC