Add build/.
Add build/platforms/.
Add build/toolchains/.
Added gcc.
Added clang.
Added msvc.
Add _build/.
Renamed build/toolchains/* into build/toolchains/*.d.
Added `VCInstallDir.sh` and `WindowSdkDir.sh` helpers to MinGW.
Add `available` helper to MSVC.
[msvc] Also write truthy/falsey values when querying for toolchain availability.

This simplifies the usage of `build/toolchains/msvc.d/available` a lot:
[windows] Check for toolchain availability in `Makefile.windows`.

It should probably fallback to the GNU Compiler Collection, but whatever.
[windows] Check the host platform when targeting `windows`.

Cross-compilation is not supported currently, and probably won't be ever.
[build] Add non-specific makefile.

Configuration-less compilation out of the box :thumbsup:
[build] Add host architecture detection script.
[build] Added host platform detection script.
[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.