First step of comment style refactoring to doxygen-style.
Buildsystem: Add support for compiling with MinGW on Windows

Tested with:
- Windows 8 x64
- MySQL 5.5.30 win32
- OpenSSL 1.0.1c (32 bits)
- No PCH
- MinGW with GCC 4.7.0

TODO:
- Fix compile/link with PCH enabled
- Fix compile with WheatyExceptonionReport enabled (ignored for now)
- Fix compile of .rc files (ignored for now)
- Test with more platforms
Merge pull request #9385 from thomas007788/Doxygen

First step of comment style refactoring to doxygen-style.
Core: Fix warnings for msvc
DB/Misc: Fix some db startup errors
Core/SmartAI: Implement SMART_ACTION_SUMMON_CREATURE_GROUP (107)