Makefile: escape $ used inside eval as shell variable

[apjanke]

Follows up on #383, which introduced a build problem on OS X.

In an eval in a Makefile, shell variables need to have their $ escaped as $$ so they're expanded by the called shell instead of by make.

Currently, the build is breaking for me like this.

[~/local/opp/git/git-extras on ⇄ master]
$ make PREFIX=/tmp/git-extras install
usage: mktemp [-d] [-q] [-t prefix] [-u] template ...
       mktemp [-d] [-q] [-u] -t prefix
... installing bins to /tmp/git-extras/bin
... installing man pages to /tmp/git-extras/share/man/man1
usage:  chmod [-fhv] [-R [-H | -L | -P]] [-a | +a | =a  [i][# [ n]]] mode|entry file ...
    chmod [-fhv] [-R [-H | -L | -P]] [-E | -C | -N | -i | -I] file ...
make: *** [install] Error 1
[✘ ~/local/opp/git/git-extras on ⇄ master]
$