Follow-up, get rid of even more NOMINMAX warnings

src/libslic3r/GCode/PostProcessor.cpp

@@ -16,7 +16,9 @@
 
 // The standard Windows includes.
 #define WIN32_LEAN_AND_MEAN
+#ifndef NOMINMAX
 #define NOMINMAX
+#endif
 #include <Windows.h>
 #include <shellapi.h>
 

src/libslic3r/Preset.cpp

@@ -7,7 +7,9 @@
 
 #ifdef _MSC_VER
     #define WIN32_LEAN_AND_MEAN
+#ifndef NOMINMAX
     #define NOMINMAX
+#endif
     #include <Windows.h>
 #endif /* _MSC_VER */
 

tests/libslic3r/test_png_io.cpp

@@ -1,4 +1,7 @@
+#ifndef NOMINMAX
 #define NOMINMAX
+#endif
+
 #include <catch2/catch.hpp>
 
 #include <numeric>