This allows finding missing includes for `.c` files (this is important
for all the cases where we generate the includes during the build
process).

We're using GCC's `-MM` `-MG` options and iterate as long as we get
new includes. Since this would return all includes verbatim from the
`#include`, we check which ones are actually generated and what are
their final paths.

Note: this is currently applied only to `.c` files and does not (yet?)
work for `.hs` files (there are issues with things like ifdefs for
package versions that cause GCC to error out).

Signed-off-by: Michal Terepeta <michal.terepeta@gmail.com>