- Let the build environment decide about the toolchain to be used, in case
  there are special tweaks and paths to be considered. Just provide some
  defaults, in case the values are undefined.
- py-version.sh does not need any bash specific features.
- Use libdl only on Linux for now. FreeBSD provides dl*() calls from libc.
Merge pull request #672 from marcusva/makefile

toolchain fixes to enable cross compatibility