Remove HAVE_W32API_H definition and checks

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.

build/msw/config.gcc

@@ -138,11 +138,6 @@ CFG ?=
 # it if SHARED=1 unless you know what you are doing. [dynamic,static]
 RUNTIME_LIBS ?= dynamic
 
-# Set the version of your Mingw installation here.
-#     "3" ...... this is for Mingw 2.0 or newer (comes with gcc3)
-#     "2.95" ... for Mingw 1.1 or any of the older versions [3,2.95]
-GCC_VERSION ?= 3
-
 # Windows resource compiler to use, possibly including extra options.
 # For example, add "-F pe-i386" here if using 64 bit windres for 32 bit build. 
 WINDRES ?= windres