Remove a compilation warning on windows.

Only define NOMINMAX if it is not already defined.

Thanks to Pierre Moulon for this fix.

Change-Id: Ia5dc0f5ff2afe10e4c7e97a57f54297d82052b21

internal/ceres/mutex.h

diff --git a/internal/ceres/mutex.h b/internal/ceres/mutex.h
index 0c48ed3..564c39f 100644
--- a/internal/ceres/mutex.h
+++ b/internal/ceres/mutex.h
@@ -112,7 +112,9 @@
 // To avoid macro definition of ERROR.
 # define NOGDI
 // To avoid macro definition of min/max.
-# define NOMINMAX
+# ifndef NOMINMAX
+#   define NOMINMAX
+# endif
 # include <windows.h>
   typedef CRITICAL_SECTION MutexType;
 #elif defined(CERES_HAVE_PTHREAD) && defined(CERES_HAVE_RWLOCK)