- cmake: remove zintconfig.h.in for now as incompatible with MSVC

project builds (will add back in future if go fully CMake)
- NO_PNG -> ZINT_NO_PNG and new API function `Zint_NoPng()` to
  determine if no PNG support in libzint; replace use in GUI with
  backend_qt method `noPng()`

backend_qt/backend_vc8.pro

@@ -13,7 +13,7 @@ INCLUDEPATH += ../backend d:\\opt\\include
 
 DEFINES +=  _CRT_SECURE_NO_WARNINGS _CRT_NONSTDC_NO_WARNINGS ZINT_VERSION=\\\"$$VERSION\\\" include=""
 
-!contains(DEFINES, NO_PNG) {
+!contains(DEFINES, ZINT_NO_PNG) {
     SOURCES += ../backend/png.c
 }