Add -Wc++-compat option, and eliminate corresponsing warnings

[nijtmans]

Recently, Tcl 9.1 (still in development) started to include utf8proc for doing its Unicode character conversions. Since Tcl is normally built with the option -Wc++-compat, this produces a lot of compiler warnings.

This resulted in the following bug report to Tcl:
https://core.tcl-lang.org/tcl/tktview/9e4c412f64

The cause of this warnings are that in C++ any operation (like '|') on an enum (like utf8proc_option_t) converts the type to an integer, which then cannot be used as an enum. This can be handled by an additional typecast, converting the integer back to the expected utf8proc_option_t.

It's unfortunate that C++ operates this way, and clang 21 (among others) warns for this with the option -Wc++-compat

Those proposed changes eliminate the warnings, and add a -Wc++-compat option in the Makefile

[eschnett]

More warnings is good, and the respective necessary boilerplate is limited. LGTM.

[nijtmans]

More warnings is good, and the respective necessary boilerplate is limited. LGTM.

Thanks @eschnett. So who has the authority to decide that this can be merged? And who can merge it?

Anything more that needs to be done?