[CMake] third party library dependencies

[Jed Brown]

On Fri, 18 Dec 2009 10:19:05 +0100, "Marcel Loose" <loose at astron.nl> wrote:
> Hi Roman,
> 
> Not in a portable way. I'm not too familiar with Windows, but on Linux
> you can do this when libA is a shared library that has its dependency on
> libB linked in (e.g. ldd libA.so will tell you this). When linking in
> static libraries you're out of luck. 

With shared libraries, you need not and *should not* explicitly link
recursive dependencies.  If you have both static and shared libraries,
the output of ldd could be used to find the recursive deps needed to
link statically.  This sucks and the logic to determine whether to put
recursive deps in FOO_LIBRARIES ends up going in FindFoo.cmake which is
insane.  FWIW, pkg-config has Libs.private for this purpose.

Finally, it would be nice to easily associate a symbol with a call to
find_library.  That is, suppose libA links to libB and libC, but libA
never exposes libB or libC to users.  To do the right thing (without
abusing ldd), FindA.cmake needs to try linking with just -lA (which will
work with all shared libs), then try with -lA -lB and -lA -lC before
falling back to -lA -lB -lC (which is required when all libs are
static).  A better way which does not have exponential complexity would
be to declare that the symbol "B_Foo" belongs with a libB and "C_Bar"
belongs with a libC.  So when linking with -lA fails, libB would be
added exactly when B_Foo is undefined.

Jed