Changes

When cross-compiling, assume strptime() is POSIX unless told otherwise.

But issue a warning when making that assumption. Add new option
FORCE_COMPAT_STRPTIME to force the use of the compat version
when cross-compiling and the target platform strptime() is not
POSIX-compliant. Poster children for the latter are BSD platforms,
including MacOS, where %t is not handled POSIXly.

Fix #472
 (commit: fc62f88)