Cherry-pick for 8.2.1: Link to libdl.so to avoid linking issues with mpich on centos7 (#856) (#862)

* Link to libdl.so to avoid linking issues with mpich on centos7 (#856)

The main coreneuron library needs to be linked to libdl.so
only in case of dynamic mpi build. But on an old system
like centos7, we saw mpich library requires explicit
ink to `libdl.so`.

See neuronsimulator/nrn-build-ci#51

* update PR template for 8.2 release

Author: pramodk

.github/pull_request_template.md

@@ -39,4 +39,4 @@ nrnivmodl mod
      These values for NEURON, nmodl and Spack are the defaults and are given
      for illustrative purposes; they can safely be removed.
 -->
-CI_BRANCHES:NEURON_BRANCH=master,NMODL_BRANCH=master,SPACK_BRANCH=develop
+CI_BRANCHES:NEURON_BRANCH=release/8.2,NMODL_TAG=0.4,SPACK_BRANCH=develop

coreneuron/CMakeLists.txt

@@ -169,13 +169,15 @@ if(CORENRN_ENABLE_MPI AND NOT CORENRN_ENABLE_MPI_DYNAMIC)
   target_link_libraries(coreneuron ${MPI_CXX_LIBRARIES})
 endif()
 
+# ~~~
+# main coreneuron library needs to be linked to libdl.so only in case of dynamic mpi build. But on
+# old system like centos7, we saw mpich library require explici link to libdl.so. See
+# https://github.com/neuronsimulator/nrn-build-ci/pull/51
+# ~~~
+target_link_libraries(coreneuron ${CMAKE_DL_LIBS})
+
 # this is where we handle dynamic mpi library build
 if(CORENRN_ENABLE_MPI AND CORENRN_ENABLE_MPI_DYNAMIC)
-  # ~~~
-  # main coreneuron library needs to be linked to libdl.so and
-  # and should be aware of shared library suffix on different platforms.
-  # ~~~
-  target_link_libraries(coreneuron ${CMAKE_DL_LIBS})
 
   # store mpi library targets that will be built
   list(APPEND corenrn_mpi_targets "")