Replace uses of __clang__ with the proper piranha macro in the tests.
type_traits: disable a test that fails on GCC < 4.8.3.
convert_to: restore the use of static_cast<> for the default implementation.

This apparently allows us to sidestep the problem of libc++ not recognizing explicit
cast operator in its type traits.
type_traits: change the way safe_abs_sint is implemented, use a constexpr function instead.

This works around a compiler bug in OSX, and looks nicer too.
kronecker_array: remove a unneeded overflow check, add some clarifications in the comments
and minor doc improvements.
ka: in the test, print out the limits structure for the default ka type.
environment: check if MPFR was built thread-safe in the init routine.
environment: make the shutdown flag an atomic variable.
ka: some minor comment fixes.
km: replace boost integer traits with numeric_limits.
Merge branch 'development' into quadmath