This enables us to distinguish in between builds which are triggered by a
subcmake call from the main swift cmake file and one from a user who is trying
to compile the swift benchmark suite against a misc swift installation/use the
ninja file by hand.
I recently broke the out of tree build by mistake [its fixed now ; )]. This
inspired me to make it easy to test this behavior by adding support to
build-script/cmake/etc for running an external benchmark build via
AddExternalProjects.
Now I can just call build-script with --build-external-benchmarks and thats it!
It should just work! It already helped me to avoid breaking the external build
twice!
I hope that eventually we get this on a bot to make sure it keeps working [or
even added to the smoke tests ; )].
*NOTE* This is disabled by default so it will not affect normal builds.
*NOTE* This just builds the external benchmarks. There is an rpath issue that
prevents you from running them (the benchmarks have the rpath set as if they are
next to the stdlib, but they are not. This can be fixed in a few different ways,
but I do not have time to finish implementing it = (. But this commit is a good
first step and at least detects build errors.
