[CMake] link errors using OpenMP

[Matthew Woehlke]

When I write a simple program using OpenMP, with a CMakeLists.txt like:

find_package(OpenMP REQUIRED)
add_definitions(${OpenMP_CXX_FLAGS})
add_executable(foo foo.cpp)

I get link errors:

foo.cpp: undefined reference to `omp_get_num_threads'
foo.cpp: undefined reference to `omp_get_thread_num'
...etc.

This is caused by the .o containing references to OMP methods, but the 
linker does not link to the platform OpenMP library (e.g. libgomp.so for 
gcc; similar issues seen with icc).

Does anyone have a preferred fix for this?

Ideas:
- find the appropriate library, set OpenMP_LIBRARY
- add OpenMP_C[XX]_FLAGS to target's LINK_FLAGS property

For the moment at least, I've solved this in my own project by creating 
the following CMake function:

function(omp_add_flags TARGET LANG)
   if(OPENMP_FOUND)
     if(NOT LANG)
       get_target_property(LANG ${TARGET} LINKER_LANGUAGE)
     endif()
     if(LANG MATCHES "C(XX)?")
       set_property(TARGET ${TARGET} APPEND
                    PROPERTY COMPILE_FLAGS ${OpenMP_${LANG}_FLAGS})
       set_property(TARGET ${TARGET} APPEND
                    PROPERTY LINK_FLAGS ${OpenMP_${LANG}_FLAGS})
     else()
       message(WARNING "omp_add_flags: target '${TARGET}'"
                       " link language '${LANG}' is not supported")
     endif()
   endif()
endfunction()


This might need some tweaking, but if the general idea seems like the 
best way, I would propose adding this to FindOpenMP.cmake.

-- 
Matthew