Log Message:
libnbcompat: Predetermine printf runtime test outcomes in configure.

This way libnbcompat can be safely cross-compiled for NetBSD, as it
should be for anything NetBSD!

While here, use CONFIGURE_ENV.${OPSYS} instead of .if ${OPSYS} == and
.if ${USE_CROSS_COMPILE:U:tl} == "yes" instead of .if
!empty(USE_CROSS_COMPILE:M[yYe][eE][sS]).