Blender Git Loki

install_deps.sh: add option to force building libs instead of using packages, and limited fallback for unsuported distro.

This commit adds '--build-foo' options to force the script to build relevant libraries
instead of trying to use packages from the distribution.

In addition, it also now offers (with those '--build-foo' options) the possibility
to build libraries on distributions that are not fully supported.

This is limited, but should still help people once they have installed themselves
the basics of dependencies - boost, llvm, osl/osd etc. are not libraries that are
really easy to build.

DISCLAIMER: I did not take the 20 (or more) hours needed to test all combinations
over all distributions, and given the size of the changes, bad sneaky typos are quite
probable, so please report if you get some errors!