Florian and me addressed his comments. I merged this and added a note to the LP 
build instructions in the wiki.

I left comments on the pull request. After reading a bit more, I think
CMAKE_DL_LIBS should work on all systems and is meant to handle the different
cases (e.g., that you don't need -ldl on Windows), so using it should be sufficient.

It seems this is a requirement of CPLEX 12.8 (see last post at 
https://www.ibm.com/developerworks/community/forums/html/topic?id=ea08c6b2-560a-4b09-a0c1-faf8050a0165&ps=25 
and http://listserv.zib.de/pipermail/scip/2018-July/003470.html).

I prepared a small pull request that uses the CMAKE_DL_LIBS variable. The variable is present in CMake 2.8.3 
(https://cmake.org/cmake/help/v2.8.3/cmake.html#variable:CMAKE_DL_LIBS). I don't think the variable is needed 
on Windows since the Windows buildslave is not complaining. It could be that the CMAKE_DL_LIBS is set to the 
empty string on Windows 
(https://gitlab.kitware.com/cmake/cmake/blob/master/Modules/Platform/Windows.cmake#L19).

Pull request: https://bitbucket.org/jendrikseipp/downward/pull-requests/105

Do you think we can merge this?

Is this a requirement of OSI or CPLEX? And is this really required, or is this a
bug where CMake discovers dynamic libraries and tries to use them in a static build?

If it is a requirement of OSI (or the CPLEX component of OSI) then the code for
it should go into the FindOSI script; if it is a requirement of CPLEX, it should
go into FindCplex.

A quick search showed that CMake has the variable CMAKE_DL_LIBS that you could
use, but I didn't find which version introduced that (we currently support CMake
versions >= 2.8.3). I also don't know if that works the same on all systems,
e.g., if the library should be added on Windows as well.

Interesting to see Fast Downward mentioned on stackoverflow. :-)

I'm trying to build the planner with LP support using the versions GCC 4.8.4, 
CPLEX 12.8 and OSI 0.107.9. I succeeded to build OSI with support for CPLEX by 
following the instructions in the wiki, but additionally adding -ldl to the --
with-cplex-lib argument as described at 
https://stackoverflow.com/a/48641753/1277298 . The dl library is also needed when 
building the planner, so currently "./build.py release64" fails during linking. 
If I append -ldl to the linking command manually, linking succeeds. (Note that 
the -ldl option has to come *after* the .o files apparently, see 
https://stackoverflow.com/a/22996058/1277298) What do I have to change to ensure 
that the -ldl option is appended automatically?