Generate and install files for pkg-config.

Generate and install files for pkg-config.

Using `pkg-config' is a standard way to query for information on linking
to other packages. This is particularly helpful for libpci where there is
no simple way to know if zlib compression is built in or not. With this
.pc file, linking to libpci is simple:

$ cc `pkg-config --cflags --libs libpci` -o myapp myapp.c

Signed-off-by: Dan Nicholson <dbn.lists@gmail.com>