gdal: fix the build

Commit 28b6fb61 added explicit CC and
CXX assignments in pkgs/build-support/gcc-wrapper/setup-hook.sh.
That breaks the gdal python extension build somehow, presumably by
confusing libtool.

This appears to revert gdal to building with g++ and gcc (on linux at
least), but at least it builds this way.

Change-Id: I83fb78daa9314dc60c4f635d3f1e937a90349b8a