contrib: Fix .pc generation

Some contribs were not correctly generating their .pc files. This MR fixes:

• libasdcp which was hardcoding /usr/local
• zlib which was not correctly replacing the prefix on windows due to PREFIX being the variable used for the cross compilation tools prefix, not the installation path
• cmake based contribs could potentially use an alternate (yet equivalent) path in case of symlinks. Passing the value as a STRING typed variable ensures cmake won't change the value.

All these changes are to ensure that we use a correct prefix when processing the .pc files from contrib/src/change_prefix.sh when generating a prebuilt contribs package.

This is split from !462 (merged) and should be merged before we start rejecting failed .pc files in prebuilt packages