The previous setup failed with clang++ 3.0.

* m4/stl.m4: New file.
* configure.ac: Call AC_HEADER_UNORDERED_MAP,
AC_HEADER_TR1_UNORDERED_MAP, and AC_HEADER_EXT_HASH_MAP.
* src/misc/hash.hh: Include _config.h, and used the
SPOT_HAVE_UNORDERED_MAP, SPOT_HAVE_TR1_UNORDERED_MAP,
or SPOT_HAVE_EXT_HASH_MAP defines to decide which
file to include.