build: Windows compilation fixes

OPTIONAL changed to NOTREQUIRED (uh) because of conflict
build: globally disable warnings for secure functions on Windows

- also fixes space in forced include header
- use OpenAL Soft on Windows
Merge pull request #12 from JanSimek/linux: MSVC compilation support

build: globally disable warnings for secure functions on Windows
build: Windows compilation fixes
Hotfixing issue #9: revert "Use SDL_sound instead of libvorbisfile"

This reverts commit 17267f6a7c7a72e9b5f25aa7cdfef7a6f0c0029c.
cmake, #12: data paths are always cached -- for those, who use gui to configure
clang fix: global include file
Merge branch 'linux' of https://github.com/vayerx/shadowgrounds into linux

Conflicts:
	shadowgrounds/shadowgrounds.cpp
	sound/openal/SoundLib.cpp
Readd Shadowgrounds.cpp:

Because of some minor changes in the origin branch, I had to manually
update shadowgrounds.cpp
Readded SoundLib.cpp

During the merge with origin, this file got delted.