handle openmp deprecation - #7882

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.01%. Comparing base (813ed95) to head (9efd5de).

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##           master    #7882   +/-   ##
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  Coverage   99.01%   99.01%           
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  Files          88       88           
  Lines       17309    17309           
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  Hits        17139    17139           
  Misses        170      170           

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FI - looks like this is now flagged by CRAN (see https://www.r-project.org/nosvn/R.check/r-devel-linux-x86_64-fedora-gcc/data.table-00check.html)

• checking whether package ‘data.table’ can be installed ... [42s/99s] WARNING
  Found the following significant warnings:
  fread.c:2479:15: warning: ‘master’ construct deprecated since OpenMP 5.1, use ‘masked’ [-Wdeprecated-openmp]
  See the install log for details.

Thank you very much, this is quite handy for the impending patch release.

The master construct, which has been deprecated, has identical semantics to the masked construct with no filter clause present.

If the filter clause is not present, the construct behaves as if the parameter is a constant integer expression that evaluates to zero, so that only the primary thread executes the associated structured block

...and we don't use the filter clause; we actually want thread 0 to run the following block. Indeed, masked is new to OpenMP 5.1. Annoyingly, they don't publish a version table for _OPENMP values, but it must be November 2020 for OpenMP 5.1 and the example output corresponds to that.

@TimTaylor looks like you'll be our 100th project member! PRs created from branches inside rdatatable/data.table will work a little better than those created from external forks.

Cc: @tdhock

...this will need a NEWS item and a DESCRIPTION patch, won't it?