correctly order weakening step to avoid race

Previously we allowed test programs to be linked at the same time as
weakening is performed, rewriting the .a archives. So lets be more
explicit. We weaken after all-am (which "runs" everything including
libraries and programs), but before all target.

Makefile.am

@@ -1390,10 +1390,11 @@ endif WITH_HEAP_PROFILER_OR_CHECKER
 
 
 # Do the weakening on some exported libtcmalloc symbols.
-install-exec-local: all-local
-all-local: $(LIBS_TO_WEAKEN)
+run-weaken: all-am
 	for la in $(LIBS_TO_WEAKEN); do lib=".libs/`basename $$la .la`.a"; [ ! -f "$$lib" ] || $(WEAKEN) "$$lib"; done
 
+install-exec-local: run-weaken
+all: run-weaken
 
 # This should always include $(TESTS), but may also include other
 # binaries that you compile but don't want automatically installed.