Summary:
CMake's find modules may have target-based generator expressions, which get propagated to us when generating the pkg-config file.
CMake 3.19 introduces the TARGET argument to file(GENERATE), which allows resolving such generator expressions and will avoid future issues.
A workaround proposed by adriaandegroot is added for CMake 3.18.

Additionally changed list->string conversion to use list(JOIN) instead of string replacement.

Fixes https://github.com/facebook/folly/issues/1414.

Pull Request resolved: https://github.com/facebook/folly/pull/1433

Reviewed By: yfeldblum

Differential Revision: D23433365

Pulled By: Orvid

fbshipit-source-id: 6ef5f180e13b41f0c92137f53fd8c19ba6355dd6