These macros are available in several systems, and we should not install
the man pages for them, otherwise we might end up shadowing the system
man pages if present.
Handle the three potential system scenarios:
- system time_t is time64
- system time_t is time32 and supports time64
- system time_t is time32 and does not support time64
Add the explicit time32 and time64 functions when necessary and map
them accordingly for each of these cases.
Add a new LIBBSD_SELECT_ABI m4 macro, and use it to setup the ABI
interfaces to expose and the various variables and conditionals
to be used by the build system.
Switch to set the initial values to unknown and then set every one
of the selections explicitly by supported target.
Update comments for rationale for things to DROP, or for why some
selections are enabled.
Detect as many warnings as possible during configure and enable them
if the user did not supply any, so that any such problem can be spotted
and fixed.
