Unset CDPATH before invoking make

At least one of Android's Makefiles parses the output of `cd' during
the build process. Having a custom CDPATH envvar set changes that
output, which breaks the build. This change simply unsets CDPATH
before calling make.
Merge pull request #194 from reubenmorais/unset-cdpath

Unset CDPATH before invoking make. r=dhylands