CONFIG: update c++ standard, some compiler rules (partially addresses #2209)

- migrate to c++14 for most compilers *except* gcc.
  There are still many systems in use with gcc-4.8.5, which does not
  support c++14.

- initial rules for nvidia compilers (pgi is will soon be defunct).
  Not fully tested...

CONFIG: provide fallback value for the user directory name

- in containers may have an unset USER env variable.
  Default to 'user' to prevent ugly looking directory names.

wmake/rules/General/Gcc/c++

@@ -1,3 +1,9 @@
+#------------------------------------------------------------------------------
+# C++14 support with gcc-5 and later, but several systems (as of 2021)
+# are still using gcc-4.8.4 (centos7, suse SLES12, ...)
+#
+# NOTE if your system gcc is new enough can simply use c++14 too.
+#------------------------------------------------------------------------------
 SUFFIXES += .C .cc .cpp .cxx
 
 CC          = g++ -std=c++11