#122 per-file overrides for program

[Kenneth C. Schalk]

./generic/compile allows for per-file overrides. These
can be used to specify command line options to be used
with only a particular file (e.g. a macro definition to
change the behavior of a particular source file).

The c_like bridge supports per-file overides when
building libraries. The "leaf" function it exports has
a "per_file_ovs" parameter. It's also possible to pas
such overrides through the "progam" function to the
building of libraries by passing them in the "lib_ovs"
parameter to the "program" function or putting them in
./lib_ovs.

The "program" function does not provide any support for
per-file overides. When it calls ./generic/compile
(through the "compile_sources" function local to the
c_like bridge) it always passes the empty binding for
the per-file overrides.

The "shared_object" function has the same limitation.