Eliminated MinGW warning

Change-Id: I35a852e742cc7d678c3af40e1bcd8a4f962303ee

examples/libmv_bundle_adjuster.cc

diff --git a/examples/libmv_bundle_adjuster.cc b/examples/libmv_bundle_adjuster.cc
index dd36875..fc45139 100644
--- a/examples/libmv_bundle_adjuster.cc
+++ b/examples/libmv_bundle_adjuster.cc
@@ -104,9 +104,12 @@
 
 #include <cstdint>
 
+// NOTE MinGW does define the macro.
+#ifndef O_BINARY
 // O_BINARY is not defined on unix like platforms, as there is no
 // difference between binary and text files.
 #define O_BINARY 0
+#endif
 
 #endif