Force pkg-config to look in the local library folder.

Setting both PKG_CONFIG_PATH and PKG_CONFIG_LIBDIR should
force pkg-config to ignore system installed libraries. We
only want it to find locally compiled dependencies if the
variable OPENSCAD_LIBRARIES is set.
Move main text test cases to the heavy set and introduce a simple test.
Merge branch 'text-module' of https://github.com/t-paul/openscad into brody-text-module

Conflicts:
	tests/CMakeLists.txt