For coverage reporting on Appveyor it's necessary to set
COVERALLS_REPO_TOKEN env variable. For security reasons it's
not available when building pull requests, so don't call 'coveralls'
in that case. To compensate, always run 'coverage report'.
When using vsXX_YY target, set up paths for Visual Studio 20XX
(YY bits) by writing a temporary batch file that calls appropriate
vcvarsall.bat and calls hererocks again, passing arguments through
another temporary file because escaping special batch characters
is difficult.
Related changes:
* Default target on Windows is now vs15_32, unless gcc is in PATH
and cl.exe isn't.
* Fixed mingw LuaJIT build attempting to install lua51.lib which isn't built.
* Fixed list flatten in run(): consider unicode strings.
