openmp detection follow up

[mattdowle]

Minor follow up to #3984

• echo=>printf to pass the checkbashism 'unsafe echo with backslash'

$ checkbashisms ./configure
possible bashism in ./configure line 60 (unsafe echo with backslash):
echo "#include <omp.h>\nint main () { return omp_get_num_threads (); }" | ${CC} ${CPPFLAGS} ${PKG_CFLAGS} ${CFLAGS} ${SHLIB_OPENMP_CFLAGS} -E -xc - >/dev/null 2>&1 || R_NO_OPENMP=1;

• removed the -E too so that this line checks linking too. I tested by making a deliberate typo in the name of the omp_get call. That took much longer to get right than I expected, as the inline comment alludes.

[codecov]

Codecov Report

Merging #4157 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4157   +/-   ##
=======================================
  Coverage   99.53%   99.53%           
=======================================
  Files          72       72           
  Lines       13911    13911           
=======================================
  Hits        13847    13847           
  Misses         64       64

---

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 5a2a02b...a2b8d47. Read the comment docs.

[MichaelChirico]

nice catch by checkbashisms, echo \n always gets me, good to know such a check exists