We had to call strlen on the preprocessed source, which seemed a bit
pointless; also, we updated shader->SourceLen but not shader->Source,
which was even more confusing. Just leave both untouched.
By using a single function, the main compiler doesn't need to include
glcpp.h, which currently has a lot of details about the preprocessor
internals. In particular, this prevents the two yacc grammars from
seeing each other, which would be rather messy to sort out.