win32: add "zint_dll_vc6" sub-directory with VC6 workspace for

creating "zint.dll" compatible with Windows XP (see ticket #339)
win32: rename README and HOWTO files to uppercase and update
  "win32\README.txt"
general: suppress some VC6 warnings
README.bsd: update

frontend_qt/HOWTO_BUILD_STANDALONE_MSVC2015.txt

@@ -0,0 +1,93 @@
+Harald Oehlmann
+2025-09-26
+
+How to build qtZint.exe using:
+- QT 5.15.16 source package
+- MS Visual Studio 2015 (VC14)
+
+Build static Qt:
+---------------
+- Go to: https://www.qt.io/offline-installers
+- Download the zip "Qt 5.15.x source packages" (nearly 1 GB):
+  https://download.qt.io/official_releases/qt/5.15/5.15.16/single/qt-everywhere-opensource-src-5.15.16.zip
+- Unzip to C:\ resulting in having the source in C:\qt-everywhere-opensource-src-5.15.16\qt-everywhere-src-5.15.16
+- Install Python (https://www.python.org/downloads/windows/) and make it available within the path.
+
+- Start the VS2015 x86 native console by the start menu entry:
+  Visual Studio 2015\Visual Studio Tools\Windows Desktop Command Prompts\VS2015 x86 Native Tools-Eingabeaufforderung
+- ("Eingabeaufforderung" = "Command Prompt")
+
+- WARNING: MS Visual Studio 2015 will only work with up to Windows Kit 10.0.20348.0, which was removed in a recent update
+  (see https://developercommunity.visualstudio.com/t/Visual-Studio-update-171411-removed-th/10953401?sort=newest)
+  It can be reinstalled at https://developer.microsoft.com/en-us/windows/downloads/sdk-archive/index-legacy
+  You may need to manually reset the environment variables INCLUDE, LIB, UCRTVersion, WindowsSDKLibVersion and WindowsSDKVersion, e.g.
+
+  set "INCLUDE=C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE;C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\ATLMFC\INCLUDE;C:\Program Files (x86)\Windows Kits\10\include\10.0.20348.0\ucrt;C:\Program Files (x86)\Windows Kits\10\include\10.0.20348.0\shared;C:\Program Files (x86)\Windows Kits\10\include\10.0.20348.0\um;C:\Program Files (x86)\Windows Kits\10\include\10.0.20348.0\winrt;"
+  set "LIB=C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\LIB;C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\ATLMFC\LIB;C:\Program Files (x86)\Windows Kits\10\lib\10.0.20348.0\ucrt\x86;C:\Program Files (x86)\Windows Kits\10\lib\10.0.20348.0\um\x86;"
+  set UCRTVersion=10.0.20348.0
+  set WindowsSDKLibVersion=10.0.20348.0\
+  set WindowsSDKVersion=10.0.20348.0\
+
+- Note: if "rc.exe" not available, install a Windows Kit and update PATH (e.g.):
+  set "PATH=C:\Program Files (x86)\Windows Kits\10\bin\10.0.20348.0\x86;%PATH%"
+
+  cd C:\qt-everywhere-opensource-src-5.15.16\qt-everywhere-src-5.15.16
+  configure.bat -static -release -prefix c:\qt\5.15.16static -qt-zlib -qt-pcre -qt-libpng -qt-libjpeg -qt-freetype -opengl desktop -no-openssl -opensource -confirm-license -make libs -nomake tools -nomake examples -nomake tests -mp
+- One may set another destination folder after the -prefix option.
+- Attention, the upper command is one long line.
+  nmake
+  nmake install
+- The last 3 commands run around 3 hours.
+- Make sure, that python may be found in the path.
+- the huge source folder may be deleted to save around 6GB of space.
+
+Zint Folder structure:
+----------------------
+- zint source files in %ZH%, e.g.
+  set "ZH=%cd%\zint"
+  git clone https://git.code.sf.net/p/zint/code "%ZH%"
+-> this file is in %ZH%\frontend_qt
+
+- libpng source files in %ZH%\..\lpng
+  git clone https://git.code.sf.net/p/libpng/code "%ZH%\..\lpng"
+-> %ZH%\..\lpng\png.h exists
+
+- zlib source files in %ZH%\..\zlib
+  git clone https://github.com/madler/zlib.git "%ZH%\..\zlib"
+-> %ZH%\..\zlib\zlib.h exists
+
+- gs1-syntax-engine source files in %ZH%\..\gs1-syntax-engine
+  git clone https://github.com/gitlost/gs1-syntax-engine "%ZH%\..\gs1-syntax-engine"
+
+Build libpng:
+-------------
+  cd "%ZH%\..\zlib"
+  nmake -f win32\Makefile.msc clean
+  nmake -f win32\Makefile.msc
+  cd ..\lpng
+  nmake -f scripts\makefile.vcwin32 clean
+  nmake -f scripts\makefile.vcwin32
+
+Build gs1-syntax-engine:
+------------------------
+  cd "%ZH%\..\gs1-syntax-engine\src\c-lib"
+  nmake -f makefile.vcwin32 clean
+  nmake -f makefile.vcwin32
+
+Build zint:
+-----------
+  set QTDIR=C:\Qt\5.15.16static
+  set PATH=C:\Qt\5.15.16static\bin;%PATH%
+  set QMAKESPEC=win32-msvc
+  cd "%ZH%"
+
+  cd backend_qt
+  qmake backend_qt.pro
+  nmake clean
+  nmake release
+
+  cd ..\frontend_qt
+  qmake frontend_qt.pro
+  nmake clean
+  nmake release
+-> qtZint.exe is in the release folder