plug-ins: optionally define several float.h macros for MinGW64.

This plug-in failed to cross-build because these macros were not defined
in the `float.h` of my MinGW64 environment (and they are used in some
ilmbase headers). Just define them ourselves if they are absent. I do
this only on MinGW environment because these should really be defined on
Linux (and other UNIX-like, I guess?) and if they are not, we may have a
bigger issue.

plug-ins/file-exr/openexr-wrapper.cc

@@ -11,6 +11,18 @@
 
 #include "openexr-wrapper.h"
 
+#if defined(__MINGW32__)
+#ifndef FLT_EPSILON
+#define FLT_EPSILON  __FLT_EPSILON__
+#endif
+#ifndef DBL_EPSILON
+#define DBL_EPSILON  __DBL_EPSILON__
+#endif
+#ifndef LDBL_EPSILON
+#define LDBL_EPSILON __LDBL_EPSILON__
+#endif
+#endif
+
 #include <ImfInputFile.h>
 #include <ImfChannelList.h>
 #include <ImfRgbaFile.h>