Added ENABLE_WEBSOCKETS option for windows builds

[MonkeybreadSoftware]

I added ENABLE_WEBSOCKETS=yes option for building on Windows.
So I can enjoy getting web socket functions without patching the flag in.

[MonkeybreadSoftware]

Anyone using Windows here with configure script?
Just to verify this works fine.

[vszakats]

appveyor.sh and/or appveyor.yml might be a good place to test them in CI. E.g. this:

--- a/appveyor.sh
+++ b/appveyor.sh
@@ -94,7 +94,7 @@ elif [ "${BUILD_SYSTEM}" = 'winbuild_vs2017' ]; then
     cd winbuild
     cat << EOF > _make.bat
       call "C:/Program Files (x86)/Microsoft Visual Studio/2017/Community/VC/Auxiliary/Build/vcvars64.bat"
-      nmake -f Makefile.vc mode=dll VC=14.10 "SSL_PATH=${openssl_root_win}" WITH_SSL=dll MACHINE=x64 DEBUG=${DEBUG} ENABLE_UNICODE=${ENABLE_UNICODE}
+      nmake -f Makefile.vc mode=dll VC=14.10 "SSL_PATH=${openssl_root_win}" WITH_SSL=dll MACHINE=x64 DEBUG=${DEBUG} ENABLE_UNICODE=${ENABLE_UNICODE} ENABLE_WEBSOCKETS=yes
 EOF
     ./_make.bat
     rm _make.bat

I think enabling this in CI for testing would be useful before merging.

[bagder]

Thanks!