[CMake] ExternalProject_Add, dependencies and Make

[Ben Pope]

Hi,

I've got a number of libraries I depend on, I use ExternalProject_Add to 
./configure and build them, I set the DEPENDS field to another 
ExternalProject_Add target.

When I generate makefiles and then use -j to make parallel builds, 
multiple instances try to build the targets in parallel, making a 
complete mess and usually more often than not, breaking the configure or 
build steps.

Any idea what I'm doing wrong?

Ben