build(macOS): correctly set number of threads for building (#20)

The current value $(nproc) only works on Linux and defaults to empty string
on macos, which results in unlimited paralellism and likely a system OOM.

build-mac.sh

@@ -24,5 +24,5 @@ fi
 mkdir -p build
 pushd build
 cmake ${CMAKE_FLAGS} ..
-cmake --build . --target dolphin-emu -- -j$(nproc)
+cmake --build . --target dolphin-emu -- -j$(sysctl -n hw.ncpu)
 popd