prefer find_package for tbb (#747)

Author: pca006132

manifoldDeps.cmake

@@ -1,9 +1,12 @@
 include(FetchContent)
 include(GNUInstallDirs)
 find_package(PkgConfig)
+if(MANIFOLD_PAR STREQUAL "TBB")
+    find_package(TBB)
+endif()
 if (PKG_CONFIG_FOUND)
     pkg_check_modules(Clipper2 Clipper2)
-    if(MANIFOLD_PAR STREQUAL "TBB")
+    if(MANIFOLD_PAR STREQUAL "TBB" AND NOT TBB_FOUND)
         pkg_check_modules(TBB tbb)
     endif()
 endif()