build: Check for 32-bit time_t at configure time

To warn of Y2038 problems, and suggest use of -D__MINGW_USE_VC2005_COMPAT
on mingw32. Do this instead of the previous runtime test.

Add -D__MINGW_USE_VC2005_COMPAT to the appveyor build.

Fixes msteveb/jimtcl#145

Signed-off-by: Steve Bennett <steveb@workware.net.au>