问题
I'm trying to apply ExternalProject_Add to automate the installation of dependencies of a mid sized C project. Things were going well until I had to install a library which uses ExternalProject_Add to install one of its dependencies, which also happen to be used by my own project. It would be nice if I could avoid having to rebuild this library, but instead use the already installed version.
Is there a good way to accomplish this? Can I tell ExternalProject to download stuff only if some condition, e.g., library already installed, is not met?
来源：https://stackoverflow.com/questions/38059659/nested-externalproject-add-with-shared-dependencies