Build: Fix horribly slow compilation times

Build: Fix horribly slow compilation times

Cedric, our dear b0rker, introduced changes in the CFLAGS
generation when merging Emile. While the changes seem to make sense
at first sight (add the -I flags for the lib our new package depends on),
they were actually a terribly bad workaround.

The number of CFLAGS args would grow exponentially, slowing down libtool
a lot, which is known to be slow when it has a lot of arguments.