fix(pkg): compat.ffmpeg linux cflags += -Wno-error=incompatible-pointer-types (musl ioctl signature) (#110)

musl declares ioctl(int, int, ...) where glibc uses unsigned long, so
libavdevice/v4l2.c's SET_WRAPPERS assignment trips gcc>=14's
incompatible-pointer-types default error when a consumer cross-builds for
x86_64-linux-musl (mcpp build --target x86_64-linux-musl --static). One
flag, linux block only, downgrades it back to a warning; glibc builds are
unaffected (the code is correct for either ABI at runtime — v4l2 request
values fit in int).

Verified locally on mcpp 0.0.101 / gcc-musl 16.1.0: full compat.ffmpeg
(2282 TU) + opencv single-repo build, static link, 6/6 opencv-m tests
incl. FFmpeg mp4 videoio roundtrip.

Author: Sunrisepeak

pkgs/c/compat.ffmpeg.lua

@@ -7765,6 +7765,14 @@ static const FFOutputFormat * const muxer_list[] = {
         },
         linux = {
             cflags = {
+                -- musl: <sys/ioctl.h> declares ioctl(int, int, ...) while glibc
+                -- (and ffmpeg's v4l2 wrappers) use unsigned long — gcc >= 14
+                -- makes the pointer-type mismatch a hard error by default.
+                -- Downgrade to a warning so x86_64-linux-musl consumer builds
+                -- (mcpp build --target x86_64-linux-musl) work; no effect on
+                -- glibc builds. Verified: full opencv-m + compat.ffmpeg musl
+                -- static build + tests green with only this change.
+                "-Wno-error=incompatible-pointer-types",
                 "-DPIC",
                 "-fomit-frame-pointer",
                 "-fno-math-errno",