[src/misc.h] Fix mc_inlined_memmove() for non-GCC toolchains.

(Copy & paster error leading to syntax error.)
[src/compat.h] Wrap GdiAlphaBlend() declaration with check...

... for mingw-w64 toolchain. The wrokaround is not needed elsewhere
cand may cause compile time clashes with other toolchain's headers.
[src/theme.[hc]] "THEMESIZE" -> "enum THEMESIZE".

MSVC knows only the latter.
[GDIX] Do not rely on system header.

MSVC provides only C++ compatible GDI+ headers.
Also adapted all callsites (i.e. src/chart.c).
[VALUE] Fix order of the '__stdcall' in function prototypes.
[MISC] Moved inclusion of <math.h> to misc.h and...

.. prepend it with #define _USE_MATH_DEFINES for MSVC compatibility.
[src/resource.rc] Make resource files quoted.

Otherwise MSVC has problems with some characters in the filenames.
[COMPAT] Provide roundf() for MSVC.
[CHART] Add 'f' suffix to float constant.
[DOC] Remove MCTRL_API from generated documentation.
[COMPAT] Fix roundf() MSVC workaround by inclusing <math.h>