[CMake] "Needs Utility" macro

[John Biddiscombe]

I have a root project with N subdirs. In most Subdirs are projects which 
are all under CMake control. Some Projects use others and all is working 
fine. The root project presents options
CSCS_BUILD_PROJECT_X
CSCS_BUILD_PROJECT_Y
CSCS_BUILD_PROJECT_Z
and then ADD_SUBDIRECTORY(ProjectX) if an option to build a sub proj is 
turned on.
If a sub project depends on another subproject and it is not turned on, 
the user sees a warning and it rurns on the other subproject.

but...
One subDir contains utilities. Libraries which are mostly used by 
various projects, but some (eg GLUT/GLUI) are used by Project X, but not 
by project Y. When the user enables Project X, we must build Glut, but 
when X is off and no other X//Y/Z project uses GLUT I don't want to 
build it. As it stands at the moment, the utilities are always built, 
whether they are needed or not (because adding a GUI option for each is 
getting tiresome)

I'd like to create a Macro that each project can call which says
Project X : NEED_UTILITY(Glut Glui QtWidgets)
Project Y : NEED_UTILITY(Glut)
Project Z : NEED_UTILITY(QtWidgets)

Clearly, when only Z is built, we don't need Glut/Glui and when Y alone, 
we don't need QtWidgets etc etc. The root level project would scan the 
list of required utilites and only add_subdirectory for those that are 
required.

Has anyone done this before? (or something similar?). I don't see any 
great difficulty in implementing it, but perhaps some functionality 
already exists which I've missed which will save me doing it - By 
default CMake will build things based on dependency, but when nothing 
depends on a lib, it is still built (since it's a target). I just want 
to turn off build for non "top-level" targets. Perhaps a 
SET_TARGET_PROPERTIES(mylib, SKIP_BUILD_IF_NOT_USED_BY_ANYTHING_ELSE)

thanks

JB

-- 
John Biddiscombe,                            email:biddisco @ cscs.ch
http://www.cscs.ch/about/BJohn.php
CSCS, Swiss National Supercomputing Centre  | Tel:  +41 (91) 610.82.07
Via Cantonale, 6928 Manno, Switzerland      | Fax:  +41 (91) 610.82.82