From Martin Beckett, "n bool ReaderWriter::fileExists(const std::string& filename, const Options* /*options*/) const

Windows doesn't define F_OK and access() is in io.h, rather than stdio.h
These are fixed in FileUtils.cpp, so I copied the fix from there - might be better to move this to FileUtils header and include it here and in FileUtils.cpp?"

src/osgDB/ReaderWriter.cpp

@@ -118,6 +118,13 @@ ReaderWriter::FeatureList ReaderWriter::featureAsString(ReaderWriter::Features f
     return result;
 }
 
+#if defined(WIN32) && !defined(__CYGWIN__)
+    #include <io.h>
+#ifndef F_OK
+    #define F_OK 4
+#endif
+#endif
+
 bool ReaderWriter::fileExists(const std::string& filename, const Options* /*options*/) const
 {
 #ifdef OSG_USE_UTF8_FILENAME