# ECMOptionalAddSubdirectory

Make subdirectories optional.

    ecm_optional_add_subdirectory(<dir>)

This behaves like `add_subdirectory()`, except that it does not complain if the directory does not exist. Additionally, if the directory does exist, it creates an option to allow the user to skip it. The option will be named BUILD_<dir>.

This is useful for “meta-projects” that combine several mostly-independent sub-projects.

If the CMake variable `DISABLE_ALL_OPTIONAL_SUBDIRECTORIES` is set to `TRUE` for the first CMake run on the project, all optional subdirectories will be disabled by default (but can of course be enabled via the respective options). For example, the following will disable all optional subdirectories except the one named “foo”:

    cmake -DDISABLE_ALL_OPTIONAL_SUBDIRECTORIES=TRUE -DBUILD_foo=TRUE myproject

Since pre-1.0.0.