[make] Compile haxelib binary with hxcpp

[tobil4sk]

This avoids the dependency on neko, which means that haxe no longer has to be packaged with neko as a mandatory dependency.

Haxelib has to be run through eval first in order to install (and run) hxcpp. Due to ssl issues on eval, hxcpp has to be installed via git rather than from lib.haxe.org.

Possible since HaxeFoundation/haxelib#643 has been merged.

gencpp no longer puts hxcpp=... into Options.txt if -D no-compilation is set because that requires a haxelib path call, which is not possible if the haxelib binary hasn't been built yet. Hxcpp never uses this value anyway and it is not very useful to store it if gencpp didn't run hxcpp.

Closes #8155.