Install a pkgconfig .pc file. This allows external plugins to find
where the header file is located, where Hexchat expects to find
plugins so it can load them at startup and which version of
Hexchat is installed. The path where pkgconfig files are stored
is determined by a macro available in newer versions of pkgconfig
which can be overridden by the --with-pkgconfig= configure switch.
However, if this macro is unavailable (older versions of
pkgconfig) then m4ifdef will make it fallback to simply use
${libdir}/pkgconfig (which is the default).

	modified:   configure.ac
	modified:   share/Makefile.am
	new file:   share/pkgconfig/Makefile.am
	new file:   share/pkgconfig/hexchat-plugin.pc.in