Minor cleanups to scons files.

* Proper check for error exits when creating libraries.

* Other cleanups
Windows: Added support for ignoring installed MinGW32 when using MinGW64
Cleanup, remove commented out addition of MinGW to path, scons does that now.
Windows: Avoid loading all the tools by "default" in Scons

* Way too much tools are loaded with no point.

* This might speed up scons startup somewhat.

* Providing these tools ourselves doesn't work well, as it's
  missing the branches. Simply adjusting default to not include
  unused stuff.
Python3: Fix, closure finalization had issues with closure references to closure references.
Fix, the meta path based loader needs to provide an "is_package" method for full compatibility.
Scons cleanups and improvements

* Added /FS option for debug mode of MSVC, newer versions need that
  for parallel compilation to work.

* Added gcc-4.9 to the list of attempted compilers

* Output the actual used MSVC in --show-scons mode

* Preparations for passing a requested MSVC version as argument.
Tests: Minor improvements
Scons cleanups
MinGW64: Enhanced support for MinGW64

* Paths with spaces do not work with "windres", need to remove useless
  options with spaces that are misparsed.

* Extract the manifest from "python.exe" if necessary with that
  compiler tools too.
Python3.4: Disable warnings that occur with Python headers.
Windows: Need to take MSVC version into account for /FS option.

* Only newer MSVC have this option, older ones error out on it.

* Affects "--debug" only.
Python3.4: Added "find_spec" implementation to meta path based loader.
Minor cleanups
Changelog updates for the release.
New pre-release.