fix build of ELSI 2.6.4 by adding CMake options for compiler commands

[boegel]

(created using eb --new-pr)

fixes errors like:

CMake Error at CMakeLists.txt:59 (MESSAGE):
  Fortran compiler must be set

problem was introduced by changes in easybuilders/easybuild-easyblocks#2514

[branfosj]

#17288?

[boegel]

#17288?

Yeah, that looks like a better approach, I'll look into that PR (and close this one afterwards)

[boegel]

Test report by @boegel
SUCCESS
Build succeeded for 2 out of 2 (2 easyconfigs in total)
node3135.skitty.os - Linux RHEL 8.6, x86_64, Intel(R) Xeon(R) Gold 6140 CPU @ 2.30GHz (skylake_avx512), Python 3.6.8
See https://gist.github.com/a7c33767ee7b492a90797e598bc2f2f2 for a full test report.

[branfosj]

Closing, as we went with #17288 instead