Fix `make install` when configured with autoconf

Commit d8cf908c (config.mak.in: remove unused definitions) removed

    exec_prefix = @exec_prefix@

from config.mak.in, because nobody directly used ${exec_prefix}, but
overlooked that other autoconf definitions could indirectly expand that
variable.

For example the following snippet from config.mak.in

    prefix = @prefix@
    bindir = @bindir@
    gitexecdir = @libexecdir@/git-core
    datarootdir = @datarootdir@
    template_dir = @datadir@/git-core/templates
    sysconfdir = @sysconfdir@

is expanded to

    prefix = /home/kirr/local/git
    bindir = ${exec_prefix}/bin                             <-- HERE
    gitexecdir = ${exec_prefix}/libexec/git-core            <--
    datarootdir = ${prefix}/share
    template_dir = ${datarootdir}/git-core/templates
    sysconfdir = ${prefix}/etc

on my system, after `configure --prefix=$HOME/local/git`

and withot exec_prefix being defined there I get an error on
install:

    install -d -m 755 '/bin'
    install -d -m 755 '/libexec/git-core'
    install: cannot create directory `/libexec': Permission denied
    Makefile:2292: recipe for target `install' failed

Fix it.

Signed-off-by: Kirill Smelkov <kirr@mns.spb.ru>
Signed-off-by: Junio C Hamano <gitster@pobox.com>