platformtest.sh: Allow using separate source and build trees

Now you can call platformtest.sh using its complete path name from an
other directory which is then used for building.

See <http://www.gnu.org/software/automake/manual/html_node/VPATH-Builds.html>

Please not that the build system itself is still (re-)genrated in the
original source tree. This can be avoided by passing the "-x" switch to
the platformtest.sh script.