Pkgconf: a pkg-config replacement

**note**: this is a work in progress.

pkgconf is a drop-in replacement for pkg-config (when symlinked). It provides the same functionality while retaining a smaller footprint. It also avoids linking or bundling third party libraries which makes it a better suited tool for minimal systems.

Tested on x86 and x86_64. Speaking of portability, Freebsd switched to pkgconf july last year which at least should be an indication on testing on more architectures than the one's I've tried.

**Known issues:**
 - [ ] glib-target inherits the incorrect CFLAGS path (set libffi_CFLAGS)
 - [ ] util-linux inherits system paths since exec_prefix is missing in some `.pc.in`