If you switch between "cmake" and "autoconf" toolchain builds, and
neglect to clean out pnacl_newlib_raw/ in between, the wrong libgtest
and libgtest_main may get pulled in for the autoconf build, leading to
an assertion failure in "make check-unit".

This tweak fixes that problem by rejiggering the lib search path.

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/1099093005