Improve the build system

[Calinou]

• The SCons option system is now used for validation
• An help message can now be displayed using scons -h
• New use_mingw option to compile using MinGW on Windows
• Removed arch in favor of bits
• Renamed headers to headers_dir
• Built libraries now contain the target in the file name

Help message preview:

❯ scons -h
scons: Reading SConscript files ...
scons: done reading SConscript files.

platform: Target platform (linux|osx|windows)
    default: linux
    actual: linux

bits: Target platform bits (default|32|64)
    default: default
    actual: default

use_llvm: Use the LLVM compiler - only effective when targeting Linux (yes|no)
    default: False
    actual: False

use_mingw: Use the MinGW compiler - only effective on Windows (yes|no)
    default: False
    actual: False

target: Compilation target (debug|release)
    default: debug
    actual: debug

headers_dir: Path to the directory containing Godot headers ( /path/to/headers_dir )
    default: godot_headers
    actual: godot_headers

Use scons -H for help about command-line options.

Note that the p and a aliases were removed. I can re-add the former for consistency with the Godot build system if desired.