check if R CMD SHLIB can compile OpenMP directly

[kevinushey]

Part of #4348.

The intention here is that if the user has already modified src/Makevars such that OpenMP "just works", then that should be detected by explicitly validating that R CMD SHLIB can compile with OpenMP.

This allows data.table to compile using OpenMP for me when using the following Makevars:

CPPFLAGS     += -I/usr/local/opt/libomp/include -Xpreprocessor -fopenmp
LDFLAGS      += -L/usr/local/opt/libomp/lib -lomp

SHLIB_OPENMP_CFLAGS   =
SHLIB_OPENMP_CXXFLAGS =

Asking whether we can set those flags (or similar) in R 4.0.0 is a separate question, I think.

[jangorecki]

Thank @kevinushey for this PR.
Works fine on my local Xubuntu.
GitLab pipeline (unfortunately we don't have Mac build there yet #3326) revealed that clean up is not working good enough:

* checking if this is a source package ... NOTE
 Found the following apparent object files/libraries:
   test-omp.o
 Object files/libraries should not be included in a source package.

* checking top-level files ... NOTE
 Non-standard file/directory found at top level:
   ‘test-omp.o’

@mattdowle could you please invite Kevin to our members group, so he can push directly to Rdatatable/data.table branches?

[codecov]

Codecov Report

Merging #4374 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4374   +/-   ##
=======================================
  Coverage   99.61%   99.61%           
=======================================
  Files          73       73           
  Lines       14104    14104           
=======================================
  Hits        14049    14049           
  Misses         55       55           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update dc5e11a...3c28fa7. Read the comment docs.

[mattdowle]

Thanks, @kevinushey! As Jan suggested, I've invited you to be project member, and will add you to contributors list.