Replace nproc in macOS setup script

nproc may not be found on macOS causing boost not to install.
Instead, switch to use sysctl to determine the number of
logical CPU that nproc would have returned.

scripts/setup-macos.sh

@@ -104,7 +104,7 @@ function install_boost {
   (
     cd ${DEPENDENCY_DIR}/boost
     ./bootstrap.sh --prefix=${INSTALL_PREFIX}
-    ${SUDO} ./b2 "-j$(nproc)" -d0 install threading=multi --without-python
+    ${SUDO} ./b2 "-j${NPROC}" -d0 install threading=multi --without-python
   )
 }