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.
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