Add comments for optional modifications.

* if the system UCX is too old
* if you want to make Intel MPI work with srun via PMI2
* or if you need to avoid a floating point exception

The UCX dep could perhaps be unconditional once UCX 1.8.0 is merged.

Fixes easybuilders#10314

easybuild/easyconfigs/i/impi/impi-2019.7.217-iccifort-2020.1.217.eb

@@ -12,11 +12,24 @@ source_urls = ['https://registrationcenter-download.intel.com/akdlm/irc_nas/tec/
 sources = ['l_mpi_%(version)s.tgz']
 checksums = ['90383b0023f84ac003a55d8bb29dbcf0c639f43a25a2d8d8698a16e770ac9c07']
 
+dependencies = [
+# Uncomment this if your system UCX is too old.
+#    ('UCX', '1.8.0'),  # needed by libfabric provider MLX introduced in Intel MPI v2019.6
+]
+
 dontcreateinstalldir = True
 
 components = ['intel-mpi', 'intel-psxe', 'intel-imb']
 
 # set up all the mpi commands to default to intel compilers
 # set_mpi_wrappers_all = True
 
+modextravars = {
+# to enable SLURM integration with srun (site-specific)
+# 'I_MPI_PMI_LIBRARY': 'libpmi2.so',
+
+# set this if mpirun gives you a floating point exception (SIGFPE)
+# 'I_MPI_HYDRA_TOPOLIB': 'ipl',
+}
+
 moduleclass = 'mpi'