refactored GIT_HOME to avoid broken build

at least on OSX, building from scratch using update.sh is broken because of d-programming-language.org.git.
I refactored GIT_HOME to set it there and give it as argument to d-programming-language.org.git/posix.mak
see related pull request :
https://github.com/D-Programming-Language/d-programming-language.org/pull/311