libm: Control errno support with _IEEE_LIBM configuration parameter

libm: Control errno support with _IEEE_LIBM configuration parameter

This removes the run-time configuration of errno support present in
portions of the math library and unifies all of the compile-time errno
configuration under a single parameter so that the whole library
is consistent.

The run-time support provided by _LIB_VERSION is no longer present in
the public API, although it is still used internally to disable errno
setting in some functions. Now that it is a constant, the compiler should
remove that code when errno is not supported.

This removes s_lib_ver.c as _LIB_VERSION is no longer variable.

Signed-off-by: Keith Packard <keithp@keithp.com>