Make substitutions in internal macros like $(@F:.o=.c) work.

This should not affect our build process, as

	find /usr/src -name Makefile | xargs grep '@[DF]'

has no matches (other than FreeBSD.org email addresses :-)

PR:		bin/24377
Submitted by:	Mark Valentine <mark@thuvia.demon.co.uk>
Reviewed by:	Matthew Emmerton <matt@gsicomp.on.ca>
MFC after:	4 weeks

usr.bin/make/var.c

@@ -1599,7 +1599,7 @@ Var_Parse (str, ctxt, err, lengthPtr, freePtr)
 		    vname[1] = '\0';
 		    v = VarFind(vname, ctxt, 0);
 
-		    if (v != (Var *)NULL) {
+		    if (v != (Var *)NULL && !haveModifier) {
 			/*
 			 * No need for nested expansion or anything, as we're
 			 * the only one who sets these things and we sure don't