pass better arguments to mkdep.

pass better arguments to mkdep.
- some std= parameters are actually critical, -std=c++11 makes a huge
difference for clang.
- explicitly ask mkdep to run the C++ compiler for C++ code, as again,
some compilers will see the difference.

(this should allow us to repair make depend under src/gnu/usr.bin/clang)

okay kettenis@