xpkg: per-feature flags + per-OS features (finish opencv dnn off-linux, silence dead-glob warnings) #253

Enhancement: per-feature flags + per-OS features in xpkg descriptors

Building the 3-platform compat.opencv surfaced two related gaps in the feature model. Both are needed to fully finish opencv's off-linux dnn feature and to silence benign build warnings.

1. features.<name>.flags (per-feature per-glob flags)

Today a feature can carry sources, defines, deps — but not flags (per-glob defines/cxxflags). So a feature's group-specific compile flags (e.g. opencv's dnn feature needs mlas-specific defines **/3rdparty/mlas/** → {BUILD_MLAS_NO_ONNXRUNTIME=1, MLAS_GEMM_ONLY=1, …}) must live in the base flags, whose glob then matches no source on a base (feature-off) build:

warning: [build].flags glob '**/3rdparty/mlas/**' matched no source file

mcpp xpkg parse confirms features.dnn.flags is an unknown key (silently ignored at build). Allowing flags inside a feature (applied only when the feature's sources are present) would let the flags co-locate with their sources and eliminate the dead-glob warnings.

2. per-OS features (or per-OS sources within a feature)

A feature's sources can be platform-specific: opencv's dnn on linux/windows is x86 (AVX kernels), on macOS/arm it's NEON — a different source/flag set. Today features is a neutral top-level key with one source list, so a feature can't differ per OS. Options that would solve it: allow mcpp.<os>.features, or per-OS keys inside a feature (features.dnn.macosx.sources), or document that such features should be split into a separate per-OS xpm package.

Why

• Silences benign but noisy "matched no source file" warnings (dead feature/opencl flag-globs) — ecosystem polish.
• Unblocks opencv-dnn on macOS/windows (per-OS dnn kernels) without splitting into a second package.

Repro: compat.opencv 5.0.0 on mcpp-index main (its dnn feature + the base build warnings). Happy to provide a minimal case.