Fixed possible macro redefinition (#1892)

MinGW libstdc++ may define `NOMINMAX` unconditionally. This fixes the case when it is already defined.

examples/main/main.cpp

@@ -23,7 +23,9 @@
 #include <unistd.h>
 #elif defined (_WIN32)
 #define WIN32_LEAN_AND_MEAN
+#ifndef NOMINMAX
 #define NOMINMAX
+#endif
 #include <windows.h>
 #include <signal.h>
 #endif