clean up build & convert to SLOT-ed libbase

This includes a couple of fixes:
 - use the right compiler flag names
 - execute pkg-config twice total, and not once per compile
 - build against SLOT-ed libbase
 - drop link info that was just for libbase as that handles
   things all by itself now without bothering external users

BUG=chromium-os:16623
TEST=`emerge-x86-alex metrics` still works
TEST=`cros_run_unit_tests --board x86-alex -p metrics` passes

Change-Id: I40b9216c9e2a1edef476f9369d524e6a4bf26012
Reviewed-on: https://gerrit.chromium.org/gerrit/17704


Reviewed-by: Luigi Semenzato <semenzato@chromium.org>
Reviewed-by: Darin Petkov <petkov@chromium.org>
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>