Install-prefix when building should not be hardcoded

Even though in Makefile, it is stated that git will figure out gitexecdir at runtime based on the path to the executable, there are many output files where $(prefix) will be hardcoded. Even git --exec-path will print out $(compile_prefix)/libexec/git-core instead of using run_prefix.

Example: We are building git in Docker at a separate stage to /deploy
(since /usr/local is populated with our compiler toolchain). The final
image is assembled by copying the contents of /deploy to /usr/local.
Commands like "git submodule" will fail because of the wrong git exec
path. Searching via "grep -r /deploy" in /deploy after make install
yields many results.