Cannot build in GCC 7.x

[jng]

I tried to build FDO trunk on Ubuntu 18.04 LTS, which includes GCC 7.x but it fails due to the following:

• General: Errors of the form invalid conversion from ‘wchar_t’ to ‘wchar_t*’ in '\0' assignment due to -fpermissive. Fix: Fix up the incorrect assignments.
• MySQL Driver: The <string> header included in stdafx.h now complains about errno due to finding errno.h in the MySQL Driver directory before <cerrno>. Fix: Rather that fight with include path precedence, just rename errno.h to drv_errno.h so <string> will properly resolve errno in <cerrno>