math/R: Fix build with clang/openmp 13

[jrm]

Summary

The mgcv package is one of the recommended packages to be installed with
R. A problem with mgcv and clang/openmp 13 is that mgcv versions prior
to 1.8-37 include omp.h after including R.h. This causes a conflict
between "#define match" and an openmp pragma line, which uses match().
This problem was fixed in mgcv 1.8-37, so switch to this newer version.

PR: 258887
Reported by: dim

[jrm]

• Fix plist issue when the NLS knob is on
• Bump PORTREVISION

[jrm]

Commit log update

[dim]

Builds OK for me, also installs, and seems to run (at least very rudimentarily :) ):

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