Replace our strptime code, which is from gnulib,
with the simpler and better NetBSD implementation.
This changes the ws_strptime() stub to unconditionally use
the internal implementation. Previously it would use the
system implementation of available. This is still possible
but is opt-in, i.e., code should add the necessary #ifdefs
and assume responsability for handling non-portable formats
or providing limited functionality on some platforms.
Text import allows the user to specify the strptime()
format freely, so in that case it makes sense to use the
system's implementation, and pass the responsability
for understanding the implementation and the supported
specifiers to the user.
Only fall back to our implementation if the system libc
lacks a strptime().
João Valverde