[dunecontrol][CMake] Reintroduce evaluation of CMAKE from opts file.

CMAKE can now be set in the shell or the opts file, if both are set
the shell has higher priority.
This feature was dropped in …aef001c21e3a9 to fix FS#1518.

bin/dunecontrol

@@ -17,8 +17,10 @@ if test -z "$MAKE"; then
   MAKE=make
 fi
 
+system_default_cmake="no"
 if test -z "$CMAKE"; then
   CMAKE=cmake
+  system_default_cmake="yes"
 fi
 
 space=" "
@@ -191,6 +193,14 @@ load_opts() {
           USE_CMAKE="yes"
         fi
       fi
+      if test -n "$USE_CMAKE" && test "$system_default_cmake" = "yes"; then
+        # We use cmake for building, but CMAKE is not yet set.
+        # Check the opts file for it
+        OPTS_FILE_CMAKE="$(eval CMAKE=""; . $DUNE_OPTS_FILE; eval echo \$CMAKE)"
+        if test -n "$OPTS_FILE_CMAKE"; then
+          CMAKE="$OPTS_FILE_CMAKE"
+        fi
+      fi
     fi
   fi
   if test "x$DUNE_OPTS_FILE" != "x"; then