External plugin build support

It is currently fairly tricky to compile plugins for VLC outside of the main source tree. We have to:

• install every possibly needed headers in $(includedir)/vlc/

• provide vlc-config (or something else) to easily find the right preprocessor, compiler and linker's flags,

• make sure installed headers do not depend on something not installed,

• in particular, headers should not depend on config.h - that's difficult to fix,

• a pkgconfig file and/or a .m4 set of macros to automatically find "vlc-devel" stuff from custom configure.ac.

Ideally, one should be able to build and install plugin with only the headers and without the source tree.