compile-libvlc: export a libvlc.pc file after building

The compile-libvlc.sh script will generate a shared library called libvlc.so including libvlc, libvlccore, the plugins and the contribs.

Expose a pkg-config file per arch folder to be able to supply this to other buildsystem like meson, cmake or autoconf. The file will be located at libvlc/jni/pkgconfig/$ANDROID_ABI/libvlc.pc.