Win32: preserving binary compatibility with Windows XP - Vista.

OpenSSL 1.1.0 and above uses BCrypt if available (Windows 7 or higher).
This results in an unusable binary on older Windows versions, when building
with newer Windows SDK (such as 7.0A).  Using CFLAGS to define _WIN32_WINNT
allows to set a desired ABI and make sure the binary works with Windows XP.

To not mix with other potential CFLAGS uses, it is set in GNUmakefile.

misc/GNUmakefile

@@ -82,7 +82,8 @@ win32:
 		--with-mail						\
 		--with-stream						\
 		--with-openssl=$(OBJS)/lib/$(OPENSSL)			\
-		--with-openssl-opt="no-asm no-tests"			\
+		--with-openssl-opt="no-asm no-tests			\
+			CFLAGS=-D_WIN32_WINNT=0x0501"			\
 		--with-http_ssl_module					\
 		--with-mail_ssl_module					\
 		--with-stream_ssl_module