Makefile: allow backslashes in INSTALL_DIR (for Windows)

[nojb]

INSTALL_DIR will typically contain backslashes under Windows, so it needs to be quoted so that it is correctly handled by the shell. Fixes:

The following actions will be performed:
=== recompile 1 package
  ↻ caml-mode 4.9

<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><>  🐫
⬇ retrieved caml-mode.4.9  (cached)
[WARNING] package uninstall script failed at C:\cygwin64\bin\make.exe uninstall INSTALL_DIR=C:\Users\nojebar\dune\_opam\share/emacs/site-lisp:
          # context     2.2.1 | win32/x86_64 | ocaml.5.2.0 | https://opam.ocaml.org#f45685efec14dffa74b060edb3fcc572bfcea34c
          # path        ~\dune\_opam\.opam-switch\remove\caml-mode.4.9
          # command     C:\cygwin64\bin\make.exe uninstall INSTALL_DIR=C:\Users\nojebar\dune\_opam\share/emacs/site-lisp
          # exit-code   2
          # env-file    ~\AppData\Local\opam\log\caml-mode-27228-bceeb9.env
          # output-file ~\AppData\Local\opam\log\caml-mode-27228-bceeb9.out
          ### output ###
          # cd C:\Users\nojebar\dune\_opam\share/emacs/site-lisp && rm -f -r caml-font.el caml.el camldebug.el inf-caml.el caml-help.el caml-types.el caml-font.elc caml.elc camldebug.elc inf-caml.elc caml-help.elc caml-types.elc
          # /bin/sh: line 1: cd: C:Usersnojebardune_opamshare/emacs/site-lisp: No such file or directory
          # make: *** [Makefile:83: uninstall] Error 1

∗ installed caml-mode.4.9
Done.