Restore ability to build on platforms without snprintf()

Restore ability to build on platforms without snprintf()

C89 does not in fact define snprintf() or vsnprintf(), and we must therefore
probe for the existence of those functions before trying to use them.

khw++ for pointing out my earlier error.

This reverts part or all of each of the following commits:

13d66b05c6163c3514774d3d11da5f3950e97e98 Rely on C89 vsnprintf()
e791399041815a1a45cea3c7f277c7045b96e51b Rely on C89 snprintf()
adf7d503e55721c500f0bf66560b8f5df7966fe7 pod/perlhacktips.pod: remove some outdated portability notes