configure: use the user specified valgrind excutable for massif

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

configure

@@ -2516,7 +2516,7 @@ case "$toolchain" in
         add_ldflags -fsanitize=thread -pie
     ;;
     valgrind-massif)
-        target_exec_default="valgrind"
+        target_exec_default=${valgrind:-"valgrind"}
         target_exec_args="--tool=massif --alloc-fn=av_malloc --alloc-fn=av_mallocz"
     ;;
     valgrind-memcheck)