Require a proof of suitable snprintf(3) implementation.

Require a proof of suitable snprintf(3) implementation.

My earlier commit fbd4415 did a wrong thing because it caused a failure
to fail in "make check" on Solaris 9, whereas the right thing to do when
printf() does not work as expected would be to fail the build with a
useful error message.   Implement that by testing snprintf() in Autoconf
and CMake (assume that in a given libc implementation all functions in
the printf() family have the same level of support for conversion
specifications).  Return 18 tests from the conditional space back into
TESTLIST.