Makefile: clarify runtime relative gitexecdir

Makefile: clarify runtime relative gitexecdir

"git" built with RUNTIME_PREFIX flag turned on could figure out
gitexecdir and other paths as relative to "git" executable.

However, in the section specifies gitexecdir, RUNTIME_PREFIX wasn't
mentioned, thus users may wrongly assume that "git" always locates
gitexecdir as relative path to the executable.

Let's clarify that only "git" built with RUNTIME_PREFIX will locate
gitexecdir as relative path.

Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>