cmake: Improve compiler setup for Ubuntu 12.04

Add extra stdarg checks to compiler standard checks, and fall back to
no standard setting.  This allows GCC 4.6 to work without using
cxxstd-autodetect=OFF.

Disable warnings which don't work nicely with GCC 4.6
by default (-Wmissing-prototypes).
cmake: Add BigTIFF check
cpp: tiff: Add workarounds for lack of BigTIFF support
cpp: tiff: Handle TIFFFindField searching tags with multiple datatypes