update some hints mechanics for 2020

Allow sharing of common code between different hints files
through use of: #-INCLUDE

new folder created: sys/unix/hints/include
new hints include files:
    sys/unix/hints/include/multiw-1.2020
    sys/unix/hints/include/multiw-2.2020

structure the early parts of sys/unix/hints/linux.2020 and
sys/unix/hints/macOS.2020 consistently, and utilize #-INCLUDE multiw-1.2020
and #-INCLUDE multiw-2.2020 in them. That will allow the Makefile lines
that they contain to be maintained in a single place.

sys/unix/hints/include/multiw-1.2020

@@ -0,0 +1,37 @@
+#------------------------------------------------------------------------------
+# NetHack 3.7  multiw-1.2020 $NHDT-Date: 1597332785 2020/08/13 15:33:05 $  $NHDT-Branch: NetHack-3.7 $
+
+# 1. Which windowing interface(s) should be included in this binary?
+# One or more of these can be manually uncommented and/or can be specified
+# on the 'make' command line.  If none are enabled, tty will be used.
+#WANT_WIN_TTY=1
+#WANT_WIN_CURSES=1
+#WANT_WIN_X11=1
+#WANT_WIN_QT=1
+
+# 2. What is the default window system?
+# Exactly one of these can be manually uncommented and/or can be specified
+# on the 'make' command line.  If none is enabled, the first among
+# WANT_WIN_{tty,curses,X11,Qt} that is enabled will become default.
+#WANT_DEFAULT=tty
+#WANT_DEFAULT=curses
+#WANT_DEFAULT=Qt
+#WANT_DEFAULT=X11
+
+# 3. compiler detection or optional override
+CCISCLANG := $(shell echo `$(CC) --version` | grep clang)
+ifeq "$(CCISCLANG)" ""
+CXX=g++ -std=gnu++11
+else
+CXX=clang++ -std=gnu++11
+endif
+# if you want to override the compiler detection just carried out
+# uncomment one of the following pairs as desired.
+#CC= gcc
+#CXX= g++ -std-gnu++11
+#
+#CC= clang
+#CXX=clang++ -std=gnu++11
+
+#end of multiw-1.2020
+#------------------------------------------------------------------------------