Detect clang version in autoconf (and use it a bit)

Detect clang version in autoconf (and use it a bit)

The version is read by checking the macros __clang_major__,
__clang_minor__ and __clang_patchlevel__. Hopefully they will have
coherent values everywhere.

For now, the version is used to set warnings a bit more reasonably for
older clang versions, which do not support -Wdeprecated-register and
some others that are used in pragmas in boost.

This removes many warnings when compiling with clang 3.3.