Bug 1091118 - Part 1: Remove $topsrcdir/gcc/bin from PATH on android builds. r=gps, a=lmandel

$topsrcdir/gcc/bin was mistakenly added in bug 971841, but is not provided by
anything the tooltool manifest for android builds pulls. It however is a path
that /may/ exist in the source tree when the slave ran a linux build before.
When it does exist, the meaning of non-path-prefixed commands change,
influencing what particular compiler is used in some cases.
Bug 1091118 - Part 2: Do not use the top-level cache file for freetype2 subconfigure. r=gps, a=lmandel

We modify the environment before running freetype2 configure. When it uses
the same cache file, it stores knowledge about that environment in the cache
file. The cache file is then reused to configure in js/src, with yet again a
different environment, which makes subconfigure.py clear the cache because
of the differences.

The configure in js/src is however invoked with the same environment as the
main configure was invoked with (mostly), so without freetype2 on the way,
reusing the cache for it works as expected. In fact, it works better with the
cache because of things coming from mozconfig that are not exported.

With freetype2 on the way, as mentioned above, the cache is cleared. Without
the cache, js/src/configure does new detections with a possibly different
environment, and stores that in the cache. Until the next build, which then
uses that different cache for the top-level configure.

This results in subtle differences in the HOST_CC/HOST_CXX variables on
android builds because those variables are not exported from mozconfig,
depending on PATH, what the builder was building before, and if the build
is a clobber.

Avoiding the freetype2 subconfigure writing its environment variables change
to the top-level cache makes the cache never invalidate for js/src.