build: fix srcdir!=builddir builds

Fix the build system to support a build directory different than the
source directory:
- fix the include path to gnulib
- properly depend on and use files in the source directory
- fix the ocaml dependency calculation, making sure it picks the files
  from all the places
tests: fix srcdir!=builddir runs

Export $(srcdir) for the tests, so a test script which invokes other
script(s) can locate them properly.
build: depend on bin2s.pl for ext2init-bin.S
tests: simple test for execstack supermin

Add a simple test which uses scanelf or readelf to detect whether the
supermin executable is really not executable.

Kind of followup of commit c9f7a7998021e1cbe22a8ec325d43e2bdc3eff5a.