Since we switched from autotools to Meson in commit
94592c1406, we don't need to include
`config.h` early to properly enable large file support. Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.
This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.
Our IcuCaseFold() fallback using strxfrm() is not actually case
insensitive. This commit fixes the problem by switching to
strcasecmp(). That function is not guaranteed to support UTF-8, but
it's the best we can do in this sparse situation.
Closes#111
