[iibaranov-IG]

Fixes #437.

The shared pkg-config template hardcodes -lpthread in Libs.private and -pthread in Cflags. CMake therefore emits those Unix flags even for an MSVC build, and an installed RtAudio package fails when an MSVC consumer resolves it through pkg-config.

Make the pthread compile and link flags configurable. CMake leaves them empty for MSVC and preserves the existing values for other toolchains. Autotools explicitly supplies the existing Unix values, so its generated package metadata is unchanged.

Validation:

• CMake-generated Unix metadata still contains Libs.private: -lpthread and Cflags: -pthread ....
• CMake-generated MSVC metadata contains neither -lpthread nor -pthread.
• CMake release build succeeds on macOS/CoreAudio.
• Autotools build succeeds on Debian/ALSA; make check passes (1/1).
• Autotools-generated metadata retains the original pthread flags.
• git diff --check passes.