Support custom pkg-config executable names

When cross-compiling, it's common to have executables prefixed with
the name of the architecture you're building for,
e.g. aarch64-unknown-linux-musl-cc or x86_64-unknown-freebsd-pkg-config.
Lots of build tools support a PKG_CONFIG environment variable to
enable this use case.

With this change, I was able to successfully cross-compile and run
catgirl.

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