Add .pc file for better discoverablity

pthreadpool lacks a pthreadpool.pc pkg-config file for the build system like CMake and Meson to find it as an external dependencies.

If pthreadpool only use cmake, I can help add it. But I don't know how to work with bazel and confu

Start by inspecting the repository's CMake, Bazel, and confu build definitions to see how pthreadpool is built and installed. Determine where pkg-config metadata should be added so CMake and Meson can discover the library as an external dependency. Done means a pthreadpool.pc file is provided and installed consistently with the supported build systems.

Written by the indexing model from the issue text.