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/Nvidia/link-c++

@@ -0,0 +1,8 @@
+LINK_LIBS   = $(c++DBUG)
+
+LINKLIBSO   = $(CC) $(c++FLAGS) -shared \
+    $(FOAM_EXTRA_LDFLAGS)
+
+LINKEXE     = $(CC) $(c++FLAGS) \
+    -Xlinker --add-needed \
+    $(FOAM_EXTRA_LDFLAGS)