configure: SC2016: Expressions don't expand in single quotes

Fix shellcheck warnings. Use double quotes but escape $ to make it
look more intentional.

diff --git a/configure b/configure
index f3498e3ef2448427430e782845ee69375d936ee6..6231d2b7ed2b26f241b2afa63efdcae0ad9bf2b4 100755 (executable)
--- a/configure
+++ b/configure
@@ -491,11 +491,11 @@ else
 fi
 
 if [ -z "${EMACSLISPDIR}" ]; then
-    EMACSLISPDIR='$(prefix)/share/emacs/site-lisp'
+    EMACSLISPDIR="\$(prefix)/share/emacs/site-lisp"
 fi
 
 if [ -z "${EMACSETCDIR}" ]; then
-    EMACSETCDIR='$(prefix)/share/emacs/site-lisp'
+    EMACSETCDIR="\$(prefix)/share/emacs/site-lisp"
 fi
 
 printf "Checking if emacs is available... "