Enable MultiSSL in winbuild

[Jan-E]

This patch enables support for MultiSSL for OpenSSL and Schannel in /winbuild by

1. Removing the lines in winbuild/Makefile.vc that generate an error with multiple SSL backends
2. Add /DCURL_WITH_MULTI_SSL in winbuild/MakefileBuild.vc if both USE_WINSSL and USE_SSL are set

See https://curl.haxx.se/mail/lib-2019-04/0032.html for the discussion in the curl-library mailinglist.

[vjardin]

Ok

[jay]

I don't think it can be done this way. The other makefiles only have OpenSSL and WinSSL so that might be ok but this one allows for more than that iirc so CURL_WITH_MULTI_SSL would have to be set if any combination of 2 or more are enabled. configure.ac does it like this:

curl/configure.ac

Lines 2693 to 2713 in 521bbbe

	case "x$OPENSSL_ENABLED$GNUTLS_ENABLED$NSS_ENABLED$POLARSSL_ENABLED$MBEDTLS_ENABLED$CYASSL_ENABLED$WINSSL_ENABLED$SECURETRANSPORT_ENABLED$MESALINK_ENABLED$AMISSL_ENABLED" in
	x)
	AC_MSG_WARN([SSL disabled, you will not be able to use HTTPS, FTPS, NTLM and more.])
	AC_MSG_WARN([Use --with-ssl, --with-gnutls, --with-polarssl, --with-cyassl, --with-nss, --with-schannel, --with-secure-transport, --with-mesalink or --with-amissl to address this.])
	;;
	x1)
	# one SSL backend is enabled
	AC_SUBST(SSL_ENABLED)
	SSL_ENABLED="1"
	AC_MSG_NOTICE([built with one SSL backend])
	;;
	*)
	# more than one SSL backend is enabled
	AC_SUBST(SSL_ENABLED)
	SSL_ENABLED="1"
	AC_SUBST(CURL_WITH_MULTI_SSL)
	CURL_WITH_MULTI_SSL="1"
	AC_DEFINE(CURL_WITH_MULTI_SSL, 1, [built with multiple SSL backends])
	AC_MSG_NOTICE([built with multiple SSL backends])
	;;
	esac

[Jan-E]

There are 3 possible SSL backends that can be enabled with /winbuild:

1. USE_SSL (= OpenSSL)
2. USE_WINSSL
3. USE_MBEDTLS

The second commit in this PR reintroduces the original checks to prohibit multiple backends for the purpose of enabling CURL_WITH_MULTI_SSL

!IF ( "$(USE_SSL)"=="true" && "$(USE_WINSSL)"=="true" ) \
 || ( "$(USE_SSL)"=="true" && "$(USE_MBEDTLS)"=="true" ) \
 || ( "$(USE_MBEDTLS)"=="true" && "$(USE_WINSSL)"=="true" )
CFLAGS = $(CFLAGS) /DCURL_WITH_MULTI_SSL
!ENDIF

Tested only with the combination of USE_SSL and USE_WINSSL, but should be working for any combination with USE_MBEDTLS as well.

[jay]

Thanks