[CMake] Partial 3rd-party library management

[Rolf Eike Beer]

> In order to maximize our ability to rebuild an exact copy of a previous
> revision, our repository carries copies of numerous 3rd party libraries.
>
> However, in most of their cases we are fairly selective about which
> elements we build.
>
> Downside: "make clean && make" winds up rebuilding all the damn libraries
> :)
>
> At the same time, we have a large number of customized project-wide
> customizations, depending on which client / product version we're
> building for. So, when we are building - as opposed to developing - we
> need the ability to easily perform a full rebuild, including possibly
> the libraries.
>
> For a nominal development "clean" build, upto 70% of the build time is
> spent building 3rd party libraries.
>
> Right now - for simplicity - we actually assemble the 3rd libraries from
> the top level CMakeLists.txt directly (ok, simplicity and a failure on
> my part to work out how to express that "libraryXXX.a" is an output of
> "subfolderX").
>
> /The problem/
>
> Something like a source-control revert can sometimes put /our/ part of
> the code base into a state that requires a clean. However, we don't want
> to cause the libraries to rebuild /except/ when someone changes
> compilation flags or forces a rebuild of the libraries somehow else
> (e.g. a "cleanall" target).
>
> Is there a way to do this with cmake? Or is this problem only because I
> haven't (yet) split these libraries into their own CMakeLists files?
>
> The simplest and therefore easiest complete example I have is:
>
> 8x --- snip --- x8
>    add_library(ircclient SHARED libircclient/src/libircclient.c)
>    set_property(TARGET ircclient PROPERTY COMPILE_DEFINITIONS
> IN_BUILDING_LIBIRC)
>    link_directories( ${Project_BINARY_DIR}/ircclient )
> 8x --- snip --- x8

No, you don't want this link_directories() here. You later do
target_link_libraries(something ircclient), which will make CMake fiddle
out the link directory and it's ordering on it's own. Explicitely
specifying link_directories() is almost always wrong and just adding pain.

My way to solve your general problem: have one top level CMakeLists.txt
that just collects the various libraries and then decend into your source
tree:

add_subdirectory(foreign_libA)
add_subdirectory(foreign_libB)
add_subdirectory(our_stuff)

If you want to get rid of your stuff:

cd ~/buildtree/our_stuff && make clean && make

Eike