Export path to pcre-config

[bedax]

Hello, I have a crate that needs to link to PCRE, and so it needs the location of the compiled pcre-config command. Just adding println!("cargo:config={}", path) is enough, but this would be needed in both the case where pkg-config finds it, and where it's compiled. I'm happy to submit a pull request for this, and likely will in the next few days, if you are open to pull requests?

Thanks