Git 2.18: RUNTIME_PREFIX... is it working?

I was excited to see the RUNTIME_PREFIX for POSIX systems patchset go by earlier this year. Although I didn't see any mention of it being included in the 2.18.0 release notes, it does appear that it was merged in for this release.

Has anyone else tried to get it working? It doesn't appear to be working properly for me so I'm not sure if I'm supposed to be doing something different... I didn't see any documentation on it.

Basically what happens is that I run configure with --prefix=/my/install/path --with-gitconfig=etc/gitconfig --with-gitattributes=etc/gitattributes.

Then I run make with RUNTIME_PREFIX=YesPlease.

When I look in the makefile, I see that the make variable gitexecdir is initially properly set to libexec/git-core which is what I expect.

However, later in the makefile we include the config.mak.autogen file, which was generated from config.mk.in by configure. In the .in file we have this:

 gitexecdir = @libexecdir@/git-core

After configure gets done with it, this becomes:

 gitexecdir = ${prefix}/libexec/git-core

which is a fully-qualified path. This means that exec-cmd.c is compiled with -DGIT_EXEC_PATH="/my/install/path/libexec/git-core" which effectively disables RUNTIME_PREFIX, as the exec-cmd.c:system_prefix() function always returns FALLBACK_RUNTIME_PREFIX since GIT_EXEC_PATH is not a suffix of executable_dirname (once the install location has been moved).

I suppose we need to pass more configure options to reset paths; is
there information somewhere on exactly which ones should be overridden?
 For example if I try to pass configure --libexecdir=libexec to solve
the above issue, I get an error from configure:

 configure: error: expected an absolute directory name for --libexecdir: libexec

Any info on how this is supposed to work, is welcome!