Address minor issues related to OpenEXR.

- OpenEXR handling code could conceivably report an unwarranted error when handling OpenEXR files > 2 GB on some platforms (notably Windows or 32-bit systems).
- Document that use of OpenEXR 3.0.0 or later requires compiler to explicitly claim C++11 support.

Author: c-lipka

source/base/image/openexr.cpp

@@ -103,7 +103,7 @@ class POV_EXR_OStream : public Imf::OStream
 		OPENEXR_INT64 tellp()
 		{
 			POV_LONG pos = os.tellg();
-			if((int) pos == -1)
+			if(pos == (POV_LONG)-1)
 				throw POV_EXCEPTION(kFileDataErr, "Error while writing EXR output");
 			return(pos);
 		}
@@ -145,7 +145,7 @@ class POV_EXR_IStream : public Imf::IStream
 		OPENEXR_INT64 tellg()
 		{
 			POV_LONG pos = is.tellg();
-			if((int)pos == -1)
+			if(pos == (POV_LONG)-1)
 				throw POV_EXCEPTION(kFileDataErr, "Error while reading EXR file");
 			return pos;
 		}

unix/install.txt

@@ -141,9 +141,12 @@ recommanded to generate a fully-featured POV-Ray executable.
 
 - When using Boost 1.65 or above, the compiler must conform to C++11 or later.
 
-- OpenEXR version 3.0.0 or higher is currently not supported, nor even
-  properly tested for. We hope to change this in the future, but fixing it is
-  a bit more complicated.
+- Modern versions of certain libraries (notably Imath 3.0.0 or above, as used
+  by OpenEXR 3.0.0 or above) require the compiler to explicitly identify as
+  C++11-compliant (as opposed to just supporting C++11 features quietly or
+  with warnings). If you are using any such libraries, you may need to specify
+  `CXXFLAGS=...` accordingly when invoking the `configure` script (e.g.
+  `configure CXXFLAGS="-std=c++11" ...` for gcc or clang).
 
 
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