benchmark

Add `benchmark_main.pc` to link `main()` containing library (#1779)

This is similar to the addition in https://github.com/google/googletest/commit/8604c4adac40573f806cfadae44e22f8dfaf212a#diff-eb8e49bdf5e9aafb996777a4f4302ad1efd281222bf3202eb9b77ce47496c345
that added pkg-config support in GTest. Without this, users
need to manually find the library containing `main()`.