[rigtorp]

The README only documents distro-specific Make builds, and the existing CMake setup links libpcap without discovering its headers or library. This updates the build for Linux and macOS and documents one minimal CMake configure/build/install sequence.

• Require CMake 3.20, discover libpcap (including native Homebrew and custom prefixes), use target-scoped C++11/warnings, and support standard install directories.
• Enable Linux timing tests by default when Expect is available, preserve explicit BUILD_TESTING choices, and pass absolute executable/capture paths so tests work outside the source tree. Check child exit status as well as timing.
• Run CI on PRs and master pushes with pinned checkout, read-only permissions, cancellation/timeouts, Linux GCC/Clang and macOS Clang coverage, and separate build/test/install checks.
• Ignore .vscode/ and .cache/.

Validation: GCC Release and Clang Debug builds, all five timing tests under both compilers, installation and installed usage checks passed. Also verified custom dependency/install paths with spaces, custom binary directory, automatic test defaults and explicit disabling, and expected missing-dependency/unsupported-platform errors. Actionlint and git diff --check passed. Native macOS validation awaits CI.

Closes #35.