[build] Add missing dummy command for gcov-iov.h

[Eric Botcazou]

Makefile.in contains a lengthy comment explaining the strategy for timestamp 
files associated with generated files; in particular, it explains that a dummy 
command is needed for 'make' not to think the generated file is unchanged.

We experienced strange rebuilds at installation time on a recent distribution 
and, after an in-depth investigation, traced it to a disconnect between gcov-
iov.h and its timestamp file s-iov.  Adding the missing dummy command got rid 
of the problem.

Tested on x86_64-suse-linux, applied on all active branches as obvious.


2016-08-29  Eric Botcazou  <ebotca...@adacore.com>

        * Makefile.in (gcov-iov.h): Add dummy recipe.

-- 
Eric Botcazou
Index: Makefile.in
===================================================================
--- Makefile.in	(revision 239761)
+++ Makefile.in	(working copy)
@@ -2775,7 +2775,7 @@ build/gcov-iov$(build_exeext): build/gcov-iov.o
 	+$(LINKER_FOR_BUILD) $(BUILD_LINKERFLAGS) $(BUILD_LDFLAGS) \
 		build/gcov-iov.o -o $@
 
-gcov-iov.h: s-iov
+gcov-iov.h: s-iov; @true
 s-iov: build/gcov-iov$(build_exeext) $(BASEVER) $(DEVPHASE)
 	build/gcov-iov$(build_exeext) '$(BASEVER_c)' '$(DEVPHASE_c)' \
 	    > tmp-gcov-iov.h