ffbuild/common: Make deletion of templates possible

If a target to be built includes a template file, the target's .d file
includes the template file as a prerequisite; if the code were changed so
that the template file no longer exists, one would get an error from
make that it has no rule for the template file target. Therefore add a
dummy rule for template files to make deleting them possible.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

ffbuild/common.mak

@@ -107,7 +107,7 @@ COMPILE_MSA = $(call COMPILE,CC,MSAFLAGS)
 %.c %.h %.pc %.ver %.version: TAG = GEN
 
 # Dummy rule to stop make trying to rebuild removed or renamed headers
-%.h:
+%.h %_template.c:
 	@:
 
 # Disable suffix rules.  Most of the builtin rules are suffix rules,