Add RUNTIME_PREFIX to environment for portable build?

### Comment:

Would you consider adding the build flag `RUNTIME_PREFIX=True`, to make the installation of git more portable?

See discussion here: https://lore.kernel.org/all/878td8azp1.fsf@evledraar.gmail.com/T/

Background:
I use conda-pack to move environments around all the time, and I frequently don't have to run conda-unpack because many packages _"just work"_ after decompressing the archive in a new location. It would be nice if git were on this list of easy-to-move packages.

I tried building git this way, and it did seem to be portable (the install folder could be moved around on disk):
```
make configure
RUNTIME_PREFIX=True ./configure --prefix=$(pwd)/build --with-curl
make
make install
```