[duneproject,bugfix] Correctly escape variable in file creation.

A variable was not correctly escaped during creation of the file
cmake/modules/Makefile.am causing the error message:
"duneproject: line 914: datadir: command not found."

This patch now correctly escapes it.

bin/duneproject

@@ -913,7 +913,7 @@ macroname="$macroname""Macros.cmake"
 echo "- $PROJECT/cmake/modules/Makefile.am"
 cat> "$PROJECT/cmake/modules/Makefile.am" <<EOF
 MODULES = $macroname
-modulesdir = $(datadir)/dune/cmake/modules
+modulesdir = \$(datadir)/dune/cmake/modules
 dist_modules_DATA = \${MODULES}
 
 include \$(top_srcdir)/am/global-rules