Ensure path var does not contain invalid characters
Ensure path var does not contain invalid characters

I have no idea why this didn't cause issues before, but when using a
windows-style native path with backslashes, the cmake C compiler test
failed for vorbis.

abannerth/Reaping2,ishtoo/Reaping2,ishtoo/Reaping2,HalalUr/Reaping2,abannerth/Reaping2,Reaping2/Reaping2,MrPepperoni/Reaping2-1,weanti/Reaping2,MrPepperoni/Reaping2-1,HalalUr/Reaping2,Reaping2/Reaping2,weanti/Reaping2