zlib: Quote BUILDLINK_PREFIX.zlib with `:Q' operator in fake-zlib-pc target

Properly quote BUILDLINK_PREFIX.zlib via the `:Q' operator.

Pointed out by <prlw1>, thanks!

Index: builtin.mk
===================================================================
RCS file: /cvsroot/pkgsrc/devel/zlib/builtin.mk,v
retrieving revision 1.13
diff -u -p -r1.13 builtin.mk
--- builtin.mk	4 Apr 2017 08:57:33 -0000	1.13
+++ builtin.mk	7 Jun 2018 15:44:16 -0000
@@ -93,7 +93,7 @@ _FAKE_ZLIB_PC=${BUILDLINK_DIR}/lib/pkgco
 fake-zlib-pc:
 	${RUN}	\
 	sedsrc=../../devel/zlib/files/zlib.pc.in;	\
-	src=${BUILDLINK_PREFIX.zlib}/lib${LIBABISUFFIX}/pkgconfig/zlib.pc;\
+	src=${BUILDLINK_PREFIX.zlib:Q}/lib${LIBABISUFFIX}/pkgconfig/zlib.pc;\
 	dst=${_FAKE_ZLIB_PC};					\
 	${MKDIR} ${BUILDLINK_DIR}/lib/pkgconfig;\
 	if [ ! -f $${dst} ]; then	\
@@ -102,12 +102,12 @@ fake-zlib-pc:
 			${LN} -sf $${src} $${dst};			\
 		else	\
 			${ECHO_BUILDLINK_MSG} "Creating $${dst}";	\
-			${SED}	-e s,@prefix@,${BUILDLINK_PREFIX.zlib},\
-					-e s,@exec_prefix@,${BUILDLINK_PREFIX.zlib},\
-					-e s,@libdir@,${BUILDLINK_PREFIX.zlib}/lib${LIBABISUFFIX},\
+			${SED}	-e s,@prefix@,${BUILDLINK_PREFIX.zlib:Q},\
+					-e s,@exec_prefix@,${BUILDLINK_PREFIX.zlib:Q},\
+					-e s,@libdir@,${BUILDLINK_PREFIX.zlib:Q}/lib${LIBABISUFFIX},\
 					-e s,@VERSION@,${BUILTIN_VERSION.zlib},\
-					-e s,@includedir@,${BUILDLINK_PREFIX.zlib}/include,\
-					-e s,@sharedlibdir@,${BUILDLINK_PREFIX.zlib}/lib,\
+					-e s,@includedir@,${BUILDLINK_PREFIX.zlib:Q}/include,\
+					-e s,@sharedlibdir@,${BUILDLINK_PREFIX.zlib:Q}/lib,\
 				$${sedsrc} > $${dst};			\
 		fi	\
 	fi