Use pkg-config to handle library dependencies.

Use pkg-config to handle library dependencies.

The original system of library dependencies was very confusing and brittle,
so I have decided to use pkg-config instead.

Each library now has its .pc file, which is processed by genconf, so conditionals
and variable substitutions can be used. You need to specify three Makefile
rules:
	(1) libsomething.a to build the static version
	(2) libsomething.so to build the shared version
	(3) libsomething.pc for the pkg-config metadata; this target should
	    depend on .pc files of all libraries required by this library.
	    @DEPS@ in the .pc file will be substituted by the names of such
	    libraries, so you can just say "Requires: @DEPS@" to handle
	    dependencies on other Sherlock libraries.

To link with a library, just add a depedency on its .pc file. The default
rules are smart enough to run pkg-config and construct the appropriate
linker flags.

If you want to link with an external library, the LIBS variable still works.

If you want an internal library to depend on an external library, just
mention it in the linker flags in the .pc file.