Support static linking in pkg-config

Currently only `spirv-cross-c-shared.pc` is provided for shared library. Could add a `spirv-cross-c.pc` with proper `pkg-config --static` support.

Thanks,
Kacper

Start by locating the existing spirv-cross-c-shared.pc file and the build entry point that generates or installs it. Compare the current package metadata with the requested spirv-cross-c.pc target, then run pkg-config --static to verify that static linking is supported and the resulting package metadata is correct.

Written by the indexing model from the issue text.