Updated devtools.sh for better file monitor

Updated devtools.sh for better file monitor
    devtools.sh
Simplified src/main.cpp

Simplified src/main.cpp
    src/main.cpp
Changed stl library from gnustl_shared to c++_shared

Changed stl library from gnustl_shared to c++_shared
This fixed various problems, most importantly:
     1. GPLv3 license requirement, which I would need a lawyer for.
     2. C++11 stuff properly included in the std namespace.

    android/jni/Application.mk
    android/src/org/libsdl/app/SDLActivity.java
Modularized build into shared libraries

Modularized build into shared libraries

Currently only done for the desktop build, but the benefit of following it
makes it easy to apply later for the android build.

The reasons for this is twofold
    1. Faster build times
    2. More modularized and decoupled modules.
       e.g only /graphics/ is allowd to link to SDL_image
       e.g only /model/ is allowed to link to the bullet library

The dependency structure is as follows

+––––––––––––––––––––––––––––––––––––––––––––––––––––––+
|                  Common libraries                    |
|                                                      |
|    +––––––––––––+   +–––––––––––+   +––––––––––+     |
|    |    math    |   |   util    |   |  extern  |     |
|    |            |   |           |   |          |     |
|    |            |   | SDL       |   |          |     |
|    +––––––––––––+   +–––––––––––+   +––––––––––+     |
|                                                      |
+–––––––––––––––––––––––––+––––––––––––––––––––––––––––+
                          |
+–––––––––––––––––––––––––+––––––––––––––––––––––––––––+
|                                                      |
|   +––––––––––––––––+   +–––––––––––––––––––––+       |
|   |     audio      |   |     graphics        |       |
|   |                |   |                     |       |
|   | SDL, SDL_Mixer |   | SDL, SDL_image, GL  |       |
|   +––––––––––––––––+   +–––––––––––––––––––––+       |
|                                                      |
|   +––––––––––––––––+   +–––––––––+                   |
|   |      model     |   |    io   |                   |
|   |                |   |         |                   |
|   | SDL, bullet    |   | SDL     |                   |
|   +––––––––––––––––+   +–––––––––+                   |
|                                                      |
+–––––––––––––––––––––––––+––––––––––––––––––––––––––––+
                          |
+–––––––––––––––––––––––––+––––––––––––––––––––––––––––+
|                                                      |
|             +–––––––––––––––––––––––+                |
|             |         core          |                |
|             |                       |                |
|             |    all of the above   |                |
|             +–––––––––––––––––––––––+                |
|                                                      |
+––––––––––––––––––––––––––––––––––––––––––––––––––––––+

    src/SConscript
    src/extern/empty.cpp (+)
    src/math/empty.cpp   (+)

Empty empty.cpp files just to allow scons to build the libraries.
Added better/stricter warning flags to build

Added better/stricter warning flags to build
    src/Android.mk
Added devtools clean target, better logcat filters

Added devtools.sh clean target and better logcat filters.
This log tag "SWL" will be used later by the logging system to be added.
    devtools.sh
Added auto-updating revision control id header file

Added auto-updating revision control id header file

Whenever the project is compiled using ./devtools.sh or ./compile.sh,
it creates or updates the file sr/util/gitrev.h, which gives
access to a 7 char long hash of the current commit,
using `git rev-parse --short HEAD`, stored in a macro CURRENT_GIT_REV,
though this might later be moved to a static variable.

    compile.sh
    src/util/empty.cpp   (+)