Add a NVTT version check at compile-time, fixes #5757, refs #5804.

This will prevent users who have an old NVTT installed on their system
from experiencing crashes. Other NVTT issues at runtime should be fixed
by clearing the cache.

Differential Revision: https://code.wildfiregames.com/D2765
This was SVN commit r23974.

source/graphics/TextureConverter.cpp

@@ -1,4 +1,4 @@
-/* Copyright (C) 2019 Wildfire Games.
+/* Copyright (C) 2020 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -33,6 +33,18 @@
 
 #include "nvtt/nvtt.h"
 
+// We assume NVTT is recent enough to support the alpha flag in the DXT1a format. If users try to use an
+// old version of NVTT, the game will crash when trying to decode dds files generated by NVTT.
+//
+// The support was added upstream in https://github.com/castano/nvidia-texture-tools/commit/782a127071895f538c1ae49925a6e15687e3c966
+// so, in theory, 2.0.7 and newer should be enough, but all 2.0.x releases define NVTT_VERSION as 200, so
+// we can't distinguish them. NVTT_VERSION is 201 in all development versions of the 2.0.x era, so we also
+// have to exclude that value.
+#if !defined NVTT_VERSION || NVTT_VERSION == 200 || NVTT_VERSION == 201
+#error Please use NVTT 2.1.0 or newer. \
+If your system does not provide it, you should use the bundled version by NOT passing --with-system-nvtt to premake.
+#endif
+
 /**
  * Output handler to collect NVTT's output into a simplistic buffer.
  * WARNING: Used in the worker thread - must be thread-safe.