Formalize the dependent/dependency relationship for <foo>.gz.uu vs <foo>

This helps ensure that the output files are regenerated if the input files
change, after the output files have been created.

MFC after:	3 days
Sponsored by:	Dell EMC Isilon

usr.bin/mkimg/tests/Makefile

@@ -1,5 +1,7 @@
 # $FreeBSD$
 
+.PATH:	${.CURDIR}
+
 PACKAGE=	tests
 
 ATF_TESTS_SH=	mkimg_test
@@ -7,8 +9,10 @@ ATF_TESTS_SH=	mkimg_test
 SOURCES!=	cd ${.CURDIR}; echo *.uu
 ${PACKAGE}FILES+=		${SOURCES:S,.gz.uu,,g}
 
-${${PACKAGE}FILES}:
-	uudecode -p ${.CURDIR}/${.TARGET}.gz.uu | gunzip -c > ${.TARGET}
+.for f in ${${PACKAGE}FILES}
+$f: $f.gz.uu
+	uudecode -p ${.ALLSRC} | gunzip -c > ${.TARGET}
+.endfor
 
 CLEANFILES+=	${${PACKAGE}FILES}}