Makefile generator: double-appended $(LIBS) the root Makefile
Makefile generator: double-appended $(LIBS) the root Makefile

Included .mk files use $(LIBS) internally, so the last included file
that defines $(LIBS) actually appended these to all link operations,
which may fail as the relevant '-L' flags may be missing.

BUG=20014
TEST=compile chromium on Linux x86_64
Review URL: http://codereview.chromium.org/173244
old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google