mpv's meson cross file needs an Objective-C compiler

6. mpv's meson cross file needs an Objective-C compiler

What happened:

meson.build:1582:4: ERROR: 'objc' compiler binary not defined in cross file [binaries] section

Root cause: mpv's iOS VideoToolbox/GLES hardware-decode interop (video/out/hwdec/hwdec_ios_gl.m) is Objective-C, and we'd enabled the ios-gl meson feature that compiles it. meson's cross file only declared c and cpp binaries under [binaries], with no objc/objcpp entries.

Fix: since Apple's clang itself handles C, C++, and Objective-C depending on file extension, we pointed objc/objcpp at the same clang/clang++ binaries already used for c/cpp, plus matching objc_args/objcpp_args with the same -arch/-isysroot/version-min flags as the other language entries. See buildall.sh's setup_prefix().

A related gotcha we had to account for: the generated crossfile.txt lives inside the cached prefix/<platform>/ directory, which is cached across CI runs by dependency version. This particular fix didn't change any dependency version, so the existing cache key wouldn't have picked up the corrected crossfile automatically. We added a CROSSFILE_REV marker to ci.sh's cache key specifically for this kind of change (anything that alters how crossfile.txt itself is generated, independent of dependency versions), and bumped it.

Lesson: a fix to how the build is configured (not just what version of a dependency is used) can still be silently masked by a cache keyed only on dependency versions — worth checking whether a fix actually needs a cache-invalidation companion change.