Add a pkgconfig file

Add a pkgconfig file

Providing a pkg-config file makes is easy for 3rd party applications
to link against the libzfs libraries.  It also allows the libzfs
developers to modify the list of required libraries and cflags
without breaking existing applications.

The following example illustrates how pkg-config can be used:

cc `pkg-config --cflags --libs libzfs` -o myapp myapp.c

/*
 * myapp.c
 */
void main()
{
	libzfs_handle_t *hdl;

	hdl = libzfs_init();
	if (hdl)
		libzfs_fini(hdl);
}

Signed-off-by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes: #585