allow selection of libchrome version

There is no libchrome.pc file anymore -- you have to specify the
version.  Add a configure flag to allow people to set the version
to build cashew against.

BUG=chromium-os:16623
TEST=`emerge-x86-alex cashew` installs same set of files

Change-Id: I50f2222606faf3c7e4cc60af17a53cb57d807d76
Reviewed-on: https://gerrit.chromium.org/gerrit/17594
Reviewed-by: Jason Glasgow <jglasgow@chromium.org>
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
1 file changed