This new test doesn't use any Unicode in the source files, instead
encoding all UTF-8 and UTF-16 as hex.
The test script will be generated from a C file, in which both a shell
script and a batch script will be generated, as well as the Unicode file
to test.
On Cygwin, MinGW, and MSYS, we will automatically bail from the shell
script to the batch script, as cmd.exe has more reliable Unicode
support, at least on Windows 7 and later.
When the make rule is called, it first checks if `$LANG` contains UTF-8,
defining the (overridable) ENABLE_UNICODE flag. If so, it will skip the
test with a warning.
Also fixed an issue with printf in multiInclude.c causing warnings on
old MinGW versions which expect %I64, and updated the .gitignore.
