Provide artifacts with libtool and pkg-config files

Hi!

Thanks for maintaining sharp and these native libraries for libvips! They're a great help to get started. 😃

I'd like to use the native libraries provided by this project in https://github.com/criteo/JVips instead of building the libraries and maintaining their build scripts in JVips, too.

Since it's a Java project using JNI to interface with the native libraries, I need to compile a small wrapper which requires the pkg-config (*.pc) and libtool (*.la) files in order to be build.
Unfortunately, those files are removed at the end of the Linux build script:
https://github.com/lovell/sharp-libvips/blob/5007da5e52eaee573021ea71f69ec65a68c9f6bf/build/lin.sh#L284-L285

@lovell Do you think it would be possible and sensible to provider an additional artifact with just these files for people to use the pre-built libraries of sharp-libvips and link them to their own C wrappers?

Start by reading build/lin.sh around lines 284-285 and inspect how Linux artifacts are assembled and packaged. Determine how an additional artifact could retain the pkg-config (.pc) and libtool (.la) files for the pre-built libraries. Done means a published artifact contains those files and can support linking a JNI or C wrapper without rebuilding the libraries.