[docs] Add Doxyfile.
[msvc] Compile all available .c files in src/.
[msvc] Posixify paths when generating a dependency list.
Define `DOXYGEN` when generating documentation.
Make `test`, `docs`, and `clean` all PHONY.
[msvc] Don't wrap AGL_CONFIGURATION (was __AGL_CONFIGURATION__) definition in quotes.
Added agl/config.h.
Added agl/config/argument_and_error_checking.inl.
Update "How is licensed?" section in README.

Clarify that AGL can be licensed for closed platforms.
Link to detailed building guide from README.md.
Link to contribution guide from README.md.
Add --help and --usage to ./configure.
Add VERSION.
Update Sublime Text 2/3 project excludes.