configure.ac: protect string using "

configure.ac

@@ -1235,9 +1235,9 @@ else
   build_starpu_top=no
 fi
 
-AC_SUBST(STARPU_USE_TOP, test x$build_starpu_top = xyes)
-AM_CONDITIONAL(BUILD_STARPU_TOP, test x$build_starpu_top = xyes)
-if test x$build_starpu_top = xyes; then
+AC_SUBST(STARPU_USE_TOP, test "x$build_starpu_top" = "xyes")
+AM_CONDITIONAL(BUILD_STARPU_TOP, test "x$build_starpu_top" = "xyes")
+if test "x$build_starpu_top" = "xyes"; then
 	AC_DEFINE(STARPU_USE_TOP, [1], [StarPU-Top is activated])
 fi