CREATE_PROJECT: Explicitly add /opt/local/lib to support MacPorts

This should hopefully be fairly harmless for other platforms,
as well as for Homebrew-users. But it doesn't seem like CMake
adds /opt/local/lib to it's linking setup by default.

devtools/create_project/cmake.cpp

@@ -176,6 +176,9 @@ if (TARGET SDL2::SDL2)
 endif()
 include_directories(${SDL2_INCLUDE_DIRS})
 
+# Explicitly support MacPorts (hopefully harmless on other platforms)
+link_directories(/opt/local/lib)
+
 )";
 
 	for (const Feature &feature : setup.features) {