Deprecated references to pkg-config

Created by: guillemj

Hi!

At least in Debian the pkg-config implementation is being phased out in favor of the pkgconf upstream project, which is maintained, interface compatible and fixes several of long-standing issues with pkg-config.

So it might be nice to replace references to pkg-config with pkgconf. Although the tool can still be accessed as pkg-config.

There's also an example to find the pkg-config tool using AC_CHECK_PROG, but that will not work for cross-compilation, so it would be better to recommend instead PKG_PROG_PKG_CONFIG