fsort segfault with clang-11 openmp

[mattdowle]

Closes #4786

Supports OpenMP<4.5 by attempting regular dynamic schedule, checking if it is monotonic (probably is), and failing gracefully if not.
See comments in code.
By changing the #define in myomp.h to drop the monotonic: modifier (i.e. to be wrong), I confirmed the new check and error message works.

[codecov]

Codecov Report

Merging #4808 (16622da) into master (70b6b13) will increase coverage by 0.00%.
The diff coverage is 98.41%.

@@           Coverage Diff           @@
##           master    #4808   +/-   ##
=======================================
  Coverage   99.47%   99.47%           
=======================================
  Files          73       73           
  Lines       14557    14563    +6     
=======================================
+ Hits        14481    14487    +6     
  Misses         76       76           

Impacted Files	Coverage Δ
src/init.c	100.00% <ø> (ø)
src/fsort.c	95.88% <98.38%> (+0.12%)	⬆️
src/openmp-utils.c	100.00% <100.00%> (ø)

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[mattdowle]

diff coverage not 100% due to minor changes (e.g. j++ to ++j) on already uncovered lines. Need to revisit all uncovered lines anyway so not holding up this PR due to that.