Only include OpenMP headers if we are actually compiling with OpenMP

Fixes building on OSX with Clang

src/FffPolygonGenerator.cpp

@@ -2,7 +2,10 @@
 
 #include <algorithm>
 #include <map> // multimap (ordered map allowing duplicate keys)
-#include <omp.h>
+
+#ifdef _OPENMP
+    #include <omp.h>"
+#endif
 
 #include "utils/math.h"
 #include "utils/algorithm.h"

src/main.cpp

@@ -20,7 +20,9 @@
 
 #include "settings/SettingsToGV.h"
 
-#include <omp.h> // omp_get_num_threads
+#ifdef _OPENMP
+    #include <omp.h> // omp_get_num_threads
+#endif
 
 namespace cura
 {