Fix warnings about _USE_MATH_DEFINES on Windows

xs/Build.PL

@@ -78,9 +78,7 @@ if ($^O eq 'darwin') {
     }
 }
 if ($mswin) {
-    # In case windows.h is included, we don't want the min / max macros to be active.
-    # If <math.h> is included, we want the #defines to be active (M_PI etc.)
-    push @cflags, qw(-DNOMINMAX -D_USE_MATH_DEFINES);
+    push @cflags, qw(-DNOMINMAX);
 }
 
 my @INC  = qw(-Isrc/libslic3r);

xs/src/libslic3r/Fill/FillGyroid.cpp

@@ -1,7 +1,6 @@
 #include "../ClipperUtils.hpp"
 #include "../PolylineCollection.hpp"
 #include "../Surface.hpp"
-#define _USE_MATH_DEFINES
 #include <cmath>
 #include <algorithm>
 #include <iostream>

xs/src/libslic3r/libslic3r.h

@@ -4,7 +4,12 @@
 // this needs to be included early for MSVC (listing it in Build.PL is not enough)
 #include <ostream>
 #include <iostream>
+
+// Otherwise #defines like M_PI are undeclared under Visual Studio
+#ifndef _USE_MATH_DEFINES
 #define _USE_MATH_DEFINES
+#endif
+
 #include <math.h>
 #include <queue>
 #include <sstream>