[msvc] Remove useless proxies.

We lose the nice indentation in build output, but it simplifies argument
wrapping.
[build] Make the `all` target depend on `agl`.
[msvc] Prepend `$(ROOT)` to the target and source when generating a dependency list.
[build] HACK: Define `AGL_EXPORT` to nothing.
[msvc] Split up and comment targetability and toolchain checks.
[msvc] Inject Microsoft Visual Studio C/C++ into `PATH`.
[msvc] Define CFLAGS, LDFLAGS, and ARFLAGS.
[msvc] Add base rules to build `agl.lib`.