Install header files and pkg-config file for external modules

Install header files and pkg-config file for external modules

This patch installs the three header files that define the compositor
plugin interface as well as a pkg-config file.  This allows
building weston plugins outside the weston tree.  We currently don't make
any guarantees about the plugin API/ABI except that within a stable
branch we won't break it.

Conflicts:
	configure.ac
	src/compositor.h