My project uses CMake's externalproject module for pulling in dependencies not found on the host system. It was a great idea until I actually started working on the project's source code again.

Whenever I "make myproject", all dependencies are also pulled in to the make command. This means all repositories are pulled, all configure scripts are executed, and all dependencies are re-installed. Given the number and size of dependencies, this can take an annoying 20 seconds.

Is there a way to to tell CMake to build an externalproject exactly once and then ignore it if it was successful in the future?

You can just check if you have already downloaded or built it by creating a file after download or build steps and checking for that file later on.

Here is where I got this code from: https://projects.kde.org/projects/kde/superbuild/repository/revisions/master/entry/SuperBuild.cmake

# are we building a source package or should we download from the internet ?
	
if(EXISTS ${CMAKE_SOURCE_DIR}/ThisIsASourcePackage.valid )	
  set(buildFromSourcePackage TRUE)
else()
  set(buildFromSourcePackage FALSE)
endif()

Thanks! That works perfectly.