All still supported versions of MinGW/Cygwin provide w32api.h file, so
there is no need to test for it.
This allows to simplify the code, but also remove the definition of
HAVE_W32API_H from bake- and makefiles and this, in turn, allows to get
rid of extra flags in MinGW format entirely, as we don't support gcc
2.95 for which they were originally needed neither.
This undocumented "private" class was used for various windows UxTheme
functions which are available since WinXP. As wxWidgets 3.1 is XP+ it
does not make sense anymore to load the theme functions dynamically.
This library is now required when wxUSE_ACCESSIBILITY==1, but was only linked,
using a compiler-specific pragma, when using MSVC resulting in link errors
with gcc.
Fix this by adding the library to the bakefile for non-MSVC compilers and
rebaking.
