Generalise RTools config/support

[andrjohns]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

This PR makes our handling of different RTools versions a little more flexible, and reduce some of the hard-coded handling of different versions.

I've also updated the CXXFLAGS for the MSVCRT setting handling to:

        "CXXFLAGS += -Wno-nonnull -D_UCRT",
        "TBB_CXXFLAGS= -D_UCRT"

Which enables the missing definitions

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Andrew Johnson

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[rok-cesnovar]

Fantastic. Assuming the tests go green now with master merged in, this is good to be merged.

[codecov-commenter]

Codecov Report

Merging #777 (d50ab95) into master (5f61efe) will increase coverage by 0.27%.
The diff coverage is 48.48%.

❗ Current head d50ab95 differs from pull request most recent head 3b2aeca. Consider uploading reports for the commit 3b2aeca to get more accurate results

@@            Coverage Diff             @@
##           master     #777      +/-   ##
==========================================
+ Coverage   86.22%   86.50%   +0.27%     
==========================================
  Files          12       12              
  Lines        4130     4134       +4     
==========================================
+ Hits         3561     3576      +15     
+ Misses        569      558      -11     

Impacted Files	Coverage Δ
R/install.R	55.41% <46.87%> (+3.34%)	⬆️
R/utils.R	71.78% <100.00%> (-0.57%)	⬇️

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