Made compilation of *.spec.c files use -fno-builtin regardless of where they are.

diff --git a/Make.rules.in b/Make.rules.in
index 0b4fbd1..a4ef8d1 100644
--- a/Make.rules.in
+++ b/Make.rules.in

@@ -77,11 +77,15 @@
 
 # Implicit rules
 
-.SUFFIXES: .rc .res .spec .spec.c .glue.s
+.SUFFIXES:
+.SUFFIXES: .rc .res .spec .spec.c .spec.o .glue.s $(SUFFIXES)
 
 .c.o:
 	$(CC) -c $(ALLCFLAGS) -o $*.o $<
 
+.spec.c.spec.o:
+	$(CC) -c $(ALLCFLAGS) @GCC_NO_BUILTIN@ -o $*.spec.o $<
+
 .s.o:
 	$(AS) -o $*.o $<