Make Plugins Admin work with MinGW in debug mode binary

Convert path to nppPluginList.json with ::wstring2string().

Unlike MSVC [1], MinGW GCC doesn't provide file stream constructors
for wchar_t* paths [2].

 ---
 [1] https://stackoverflow.com/a/822032
 [2] https://sourceforge.net/p/mingw-w64/mailman/message/34249829

Fixes #11687, close #11701

PowerEditor/gcc/makefile

@@ -60,7 +60,7 @@ else
 BUILD_TYPE := debug
 BUILD_SUFFIX := -debug
 CXXFLAGS += -Og -g -Wall -Wpedantic -Wconversion-null
-#CPP_DEFINE += DEBUG
+CPP_DEFINE += DEBUG
 endif
 
 #

PowerEditor/src/WinControls/PluginsAdmin/pluginsAdmin.cpp

@@ -706,7 +706,11 @@ bool PluginsAdminDlg::initFromJson()
 #ifdef DEBUG // if not debug, then it's release
 	
 	// load from nppPluginList.json instead of nppPluginList.dll
+#ifdef __MINGW32__
+	ifstream nppPluginListJson(wstring2string(_pluginListFullPath, CP_UTF8));
+#else // MSVC supports UTF-16 path names in file stream constructors 
 	ifstream nppPluginListJson(_pluginListFullPath);
+#endif
 	nppPluginListJson >> j;
 
 #else //RELEASE

appveyor.yml

@@ -27,11 +27,6 @@ configuration:
     - Release
     - Debug
 
-matrix:
-  exclude:
-    - compiler: GCC
-      configuration: Debug
-
 before_build:
     - ps: |
         Write-Output "Compiler : $env:compiler"
@@ -98,6 +93,7 @@ for:
     only:
       - compiler: GCC
   install:
+    - 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;=%
   build_script: