configure: clang: explicitly state dep file and rule name in DEPFLAGS

Fixes dependency file generation with gas-preprocessor.pl and clang.
Flags copied from GCC and tested with Apple's clang from Xcode 5 and
5.1 and clang 3.2, 3.3, 3.4 on Linux.

configure

@@ -2702,7 +2702,7 @@ probe_cc(){
     elif $_cc -v 2>&1 | grep -q clang; then
         _type=clang
         _ident=$($_cc --version | head -n1)
-        _depflags='-MMD'
+        _depflags='-MMD -MF $(@:.o=.d) -MT $@'
         _cflags_speed='-O3'
         _cflags_size='-Os'
     elif $_cc -V 2>&1 | grep -q Sun; then