(MinGW-w64) Compilation errors

[Teemu Nätkinniemi]

When compiling Stellarium with MinGW-w64 the following errors occur:

[ 19%] Building CXX object plugins/Satellites/src/CMakeFiles/Satellites-static.dir/Satellite.cpp.obj
In file included from g:\cbad\mingw64\bin\../lib/gcc/x86_64-w64-mingw32/4.6.1/../../../../x86_64-w64-mingw32/include/urlmon.h:219:0,
                 from g:\cbad\mingw64\bin\../lib/gcc/x86_64-w64-mingw32/4.6.1/../../../../x86_64-w64-mingw32/include/objbase.h:346,
                 from g:\cbad\mingw64\bin\../lib/gcc/x86_64-w64-mingw32/4.6.1/../../../../x86_64-w64-mingw32/include/ole2.h:16,
                 from g:\cbad\mingw64\bin\../lib/gcc/x86_64-w64-mingw32/4.6.1/../../../../x86_64-w64-mingw32/include/windows.h:97,
                 from G:/cbad/qt-4.8.0-x64/include/QtCore/../../src/corelib/global/qt_windows.h:63,
                 from G:/cbad/qt-4.8.0-x64/include/QtCore/qt_windows.h:1,
                 from G:/cbad/qt-4.8.0-x64/include/QtOpenGL/../../src/opengl/qgl.h:58,
                 from G:/cbad/qt-4.8.0-x64/include/QtOpenGL/qgl.h:1,
                 from G:/cbad/qt-4.8.0-x64/include/QtOpenGL/QtOpenGL:5,
                 from G:/MinGWx64/msys/1.0/src/stellarium/src/core/StelTexture.hpp:27,
                 from g:/MinGWx64/msys/1.0/src/stellarium/plugins/Satellites/src/Satellite.cpp:26:
g:\cbad\mingw64\bin\../lib/gcc/x86_64-w64-mingw32/4.6.1/../../../../x86_64-w64-mingw32/include/msxml.h:624:109: error: expected ',' or '...' before numeric constant
g:\cbad\mingw64\bin\../lib/gcc/x86_64-w64-mingw32/4.6.1/../../../../x86_64-w64-mingw32/include/msxml.h:830:143: error: expected ',' or '...' before numeric constant
make[2]: *** [plugins/Satellites/src/CMakeFiles/Satellites-static.dir/Satellite.cpp.obj] Error 1
make[1]: *** [plugins/Satellites/src/CMakeFiles/Satellites-static.dir/all] Error 2
make: *** [all] Error 2

Attached patch fixes the problem by using M_PI from math.h and defines M_PI it if math.h for some reason doesn't define it.