build(cmake): improve pkg-config generation

build(cmake): improve pkg-config generation

With this patch the pkg-config generation when using the CMake build
system is improved in the following ways:

- Libs.private is now filled when needed
- The JoinPaths module is now used to join paths, leading to simpler
  code
- The .pc file is always generated, regardless of the platform, as it
  can also be consumed on Windows

Here's how the .pc file is affected by these changes, in comparison to
the one generated with the official Makefiles:

    $ diff -s lib/libzstd.pc build/cmake/build-old/lib/libzstd.pc
    15c15
    < Libs.private: -pthread
    ---
    > Libs.private:

    $ diff -s lib/libzstd.pc build/cmake/build-new/lib/libzstd.pc
    Files lib/libzstd.pc and build/cmake/build-new/lib/libzstd.pc are
    identical