Only check if compass is available through sass if sass was found.

configure.ac

@@ -94,6 +94,7 @@ if test x"${COMPASS}" == x"" ; then
 fi
 AC_SUBST(COMPASS)
 
+if test x"${SASS}" != x"" ; then
     AC_MSG_CHECKING([for compass support in sass])
     tempfile=`mktemp`
     $SASS --compass $tempfile > /dev/null 2> /dev/null
@@ -104,6 +105,7 @@ if test x"${sass_compass}" != x"0" ; then
         AC_MSG_WARN([Please install compass so it can be used using "sass --compass ...".])
     fi
     AC_MSG_RESULT([ok])
+fi
 
 AC_PATH_PROGS([AUTOPREFIXER],[autoprefixer])
 if test x"${AUTOPREFIXER}" == x"" ; then