Add timestamp-based dependency system to build_clib

[GandaG]

build_clib is sorely missing one thing in distutils - checking outdated dependencies when compiling. This makes it so that, when building a static c library, all objects are always all compiled.

The function to create the static library itself does implement something of this sort, but since all objects have been recompiled it has no practical effect (everything still runs every time).

This is a small patch designed to fix that, with a couple of qol improvements along the way. Two new keys were added to the build_info dictionary:

• obj_deps - Allows specifying extra dependencies for each source so they can be recognized as being newer than the object being compiled. Example:

setup(
...
libraries=[(
    'examplelib', 
    {'sources': 'example.c', 
     'obj_deps': {'': 'global.h',  # use an empty string to define global (for all sources) dependencies
                  'example.c': 'example.h'}
    }
)]
...
)

• cflags - This just allows the user to provide an extra set of flags to the compiler.

That is pretty much it, a simple (but important) quality of life improvement for those of us who need to compile static libs with python.

If the implementation is fine I can get started on documentation, just not sure where to put it.

[jaraco]

This seems reasonable, and I appreciate the effort put into making this first request a complete and thorough one.

I welcome subsequent improvements to the documentation. I also encourage you to see this implementation made the default in distutils (stdlib) with setuptools providing forward compatibility but eventually delegating that role back to distutils. I won't stipulate that as a requirement, however.

[GandaG]

I'll definitely look into that, time to start learning mercurial :) Thanks!