Update CI env to have GCC 12 for using c++20

1. Update Makefile to use c++20.
2. Update appveyor.yml to use MSYS2 (GCC 12).

Close #12292

PowerEditor/gcc/makefile

@@ -37,7 +37,7 @@ LEXILLA_DIRECTORY := ../../lexilla
 LEXILLA_TARGET := liblexilla.a
 
 CXX := $(CROSS_COMPILE)g++
-CXXFLAGS := -include $(GCC_DIRECTORY)/gcc-fixes.h -std=c++17 -Wno-conversion-null
+CXXFLAGS := -include $(GCC_DIRECTORY)/gcc-fixes.h -std=c++20
 RC := $(CROSS_COMPILE)windres
 RCFLAGS :=
 CPP_PATH := $(SCINTILLA_DIRECTORY)/include $(LEXILLA_DIRECTORY)/include
@@ -59,7 +59,7 @@ LDFLAGS += -s
 else
 BUILD_TYPE := debug
 BUILD_SUFFIX := -debug
-CXXFLAGS += -Og -g -Wall -Wpedantic -Wconversion-null
+CXXFLAGS += -Og -g -Wall -Wpedantic
 CPP_DEFINE += DEBUG
 endif
 

appveyor.yml

@@ -93,9 +93,17 @@ for:
     only:
       - compiler: GCC
   install:
+    - set PATH=C:\msys64\usr\bin
+    - if "%platform%" EQU "i686" set $env:MSYSTEM = 'MINGW32'
+    - if "%platform%" EQU "x86_64" set $env:MSYSTEM = 'MINGW64'
     - if "%configuration%" EQU "Debug" set DEBUG=1
-    - if "%platform%" EQU "i686" set PATH=C:\mingw-w64\i686-8.1.0-posix-dwarf-rt_v6-rev0\mingw32\bin;%PATH:C:\Program Files\Git\usr\bin;=%
-    - if "%platform%" EQU "x86_64" set PATH=C:\mingw-w64\x86_64-8.1.0-posix-seh-rt_v6-rev0\mingw64\bin;%PATH:C:\Program Files\Git\usr\bin;=%
+    # uncomment lines below to update packages to use latest gcc
+    #- bash -lc "pacman --noconfirm -Syuu"
+    #- bash -lc "pacman --noconfirm -Syuu"
+    #- if "%platform%" EQU "i686" bash -lc "pacman --noconfirm -S mingw-w64-i686-gcc mingw-w64-i686-make"
+    #- if "%platform%" EQU "x86_64" bash -lc "pacman --noconfirm -S mingw-w64-x86_64-gcc mingw-w64-x86_64-make"
+    - if "%platform%" EQU "i686" set PATH=C:\msys64\mingw32\bin;%PATH:C:\Program Files\Git\usr\bin;=%
+    - if "%platform%" EQU "x86_64" set PATH=C:\msys64\mingw64\bin;%PATH:C:\Program Files\Git\usr\bin;=%
   build_script:
     - mingw32-make -f PowerEditor\gcc\makefile
   after_build: