win32/GNUmakefile: define HAVE_WINDOWS_H

This macro was only used in test/example code before, now it is
also used in library code, but only defined automatically by
automake/cmake, so let's do the same for the standalone win32
make file.

It'd be probably better to just rely on the built-in _WIN32 macro
to detect the presence of windows.h though. It's already used
in most of libssh2 library code. There is a 3rd, similar macro
named LIBSSH2_WIN32, which might also be replaced with _WIN32.

Ref: 8b870ad771
Closes https://github.com/libssh2/libssh2/pull/266

win32/GNUmakefile

@@ -125,7 +125,7 @@ endif
 -include $(OBJDIR)/version.inc
 
 # Global flags for all compilers
-CFLAGS	= $(LIBSSH2_CFLAG_EXTRAS) $(OPT) -D$(DB) -DLIBSSH2_WIN32 # -DHAVE_CONFIG_H
+CFLAGS	= $(LIBSSH2_CFLAG_EXTRAS) $(OPT) -D$(DB) -DLIBSSH2_WIN32 -DHAVE_WINDOWS_H # -DHAVE_CONFIG_H
 LDFLAGS = $(LIBSSH2_LDFLAG_EXTRAS)
 
 ifeq ($(CC),mwcc)