Build fails using PGI compilers

[kafitzgerald]

This is somewhat of a known issue... but figured it was worth documenting if nothing else as a reminder to remove PGI from the list of supported compilers for next release and perhaps hide that build option in the configure script (it is #1 after all).

Current Behavior

Builds using PGI compilers fail because of modern Fortran features

Possible Solution

Remove statements of support for PGI from docs / FAQs

Steps to Reproduce (for bugs)

Attempt a build w/ env listed below

Your Environment

• Version of the code used: current master
• Operating System and version: Cheyenne
• Compiler and version: PGI 19.10
• Other relevant information: v5.1.x seems to build fine w/ PGI 19 compilers

[scrasmussen]

Fixed and closed by PR #502