libchdr: add build option for using system library

[bsdcode]

The added build option USE_HOST_LIBCHDR allows to build with libchdr provided by the system. The option is disabled by default.

I have no experience with building for Android, Apple or the Switch, so I don't know if it's also necessary to exclude them from using the system library.

[flyinghead]

Thanks!

[abouvier]

Why did you exclude libretro build? If USE_HOST_LIBCHDR is set, it should be enabled for every target.

[bsdcode]

I have little experience with libretro cores and how RetroArch handles this. There was no other reason other than inexperience on the libretro topic. Now on a second look I was probably wrong here, because chdr-static was always build previously.

[flyinghead]

Fixed by df78a78

[abouvier]

By the way, libretro-common also contains a copy of libchdr headers in core/deps/libretro-common/include/libchdr.
They are searched for, and can hijack the system headers because of this line:

flycast/CMakeLists.txt

Line 779 in df78a78

target_include_directories(${PROJECT_NAME} PRIVATE core/deps/libretro-common/include)