openh264: use libc++ instead of the deprecated lstdc++
This fixes issues with static build in macOS when using lstdc++ instead of -lc++
clang: warning: libstdc++ is deprecated; move to libc++ [-Wdeprecated]
ld: library not found for -lstdc++
clang: error: linker command failed with exit code 1 (use -v to see invocation)