Ask for CXX parameter for local compilation

This allows compilation with Emacs from snapcraft.
See alexmurray/emacs-snap#92

README.md

@@ -93,6 +93,12 @@ I can’t test it but on windows you can get msys2 and
 `mingw-w64-x86_64-xapian-core` and `make` should just work. Thanks to
 pRot0ta1p for reporting this.
 
+If you're using Emacs via snap you need to add these parameters:
+
+```shell
+make PREFIX=/snap/emacs/current CXX="/snap/emacs/current/usr/bin/g++-10 --sysroot /snap/emacs/current/"
+```
+
 # notdeft
 
 I owe many thanks to the author of notdeft. I don’t really know C++ or

xeft.el

@@ -254,9 +254,11 @@ let xeft recreate it."
            (locate-library "xeft.el" t)))
          (prefix (concat "PREFIX="
                          (read-string "PREFIX (empty by default): ")))
+         (cxx (concat "CXX="
+                         (read-string "CXX (empty by default): ")))
          (buffer (get-buffer-create "*xeft compile*")))
     (if (zerop (let ((inhibit-read-only t))
-                 (call-process "make" nil buffer t prefix)))
+                 (call-process "make" nil buffer t prefix cxx)))
         (progn (message "Successfully compiled the module :-D") t)
       (pop-to-buffer buffer)
       (compilation-mode)