compiler settings alignment between macOS.2020 and linux.2020

get the warning flags on linux and macOS into better alignment

move the compiler flags into hints/include/compiler.2020 so that
maintenance changes can be made there and take effect for both
linux and macOS

this will likely trigger some initial new code warnings to be
investigated on either or both platforms

sys/unix/hints/macOS.2020

@@ -16,21 +16,10 @@
 # macOS X hints file
 #
 
-# compiler flags:  CCFLAGS is used to construct a value for CFLAGS with
-#	various -I, -D, and -W settings appended below;
-#	these are the settings of most interest for an end-user build
-#	(clang doesn't support '-Og', gcc needs 4.x or later)
-CCFLAGS = -g
-#CCFLAGS = -g -Og
-#CCFLAGS = -O2
-#	Note: this is not the usual 'CFLAGS' which is used in default
-#	rules for compiling C code; specifying a value for that on the
-#	'make' command line should be avoided.
-
-# note: '#-INCLUDE' is not just a comment; multiw-1 contains sections 1 to 3
+# note: '#-INCLUDE' is not just a comment; multiw-1 contains sections 1 to 2
 #-INCLUDE multiw-1.2020
 
-# 4. If you set WANT_WIN_QT, you need to
+# 3. If you set WANT_WIN_QT, you need to
 #  A) set QTDIR either here or in the environment to point to the Qt5
 #     library installation root.  (Qt2, Qt3, Qt4 will not work)
 #  B) set XPMLIB to point to the Xpm library
@@ -50,7 +39,7 @@ ifndef LIBXPM
 LIBXPM= -L/opt/X11/lib -lXpm
 endif
 
-# 5. Other
+# 4. Other
 
 #-----------------------------------------------------------------------------
 # You shouldn't need to change anything below here (in the hints file; if
@@ -59,27 +48,10 @@ endif
 
 #-INCLUDE multiw-2.2020
 
-CFLAGS=$(CCFLAGS) -I../include -DNOTPARMDECL
+# compiler.2020 contains compiler detection and adjustments common
+# to both linux and macOS
 
-ifndef WANT_WIN_QT
-ifndef WANT_LIBNH
-#	these are normally used when compiling nethack's core
-CFLAGS+=-ansi -pedantic -Wno-long-long
-#	but -ansi forces -std=c90 for C or -std=c++98 for C++;
-#	win/Qt/qt_*.cpp compiled with C++98 semantics trigger
-#In file included from .../qt5/include/QtCore/qglobal.h:105:
-#.../qt5/include/QtCore/qcompilerdetection.h:561:6:
-#  error Qt requires a C++11 compiler and yours does not seem to be that.
-#	so we suppress -ansi when the build includes Qt
-#LIBNH's winshim requires C99 for the way it is currently coded
-endif
-endif
-# As of LLVM build 2336.1.00, this gives dozens of spurious messages, so
-# leave it out by default.
-#CFLAGS+=-Wunreachable-code
-CFLAGS+=-Wall -Wextra -Wno-missing-field-initializers -Wimplicit \
-	-Wreturn-type -Wunused -Wformat -Wswitch -Wshadow -Wwrite-strings
-CFLAGS+=-DGCC_WARN
+#-INCLUDE compiler.2020
 
 # NetHack sources control
 CFLAGS+=-DDLB