Change dll export feature to a switch.

ispc.h

@@ -612,6 +612,9 @@ struct Globals {
     /** Indicates that alignment in memory allocation routines should be
         forced to have given value. -1 value means natural alignment for the platforms. */
     int forceAlignment;
+
+    /** When true, flag non-static functions with dllexport attribute on Windows. */
+    bool dllExport;
 };
 
 enum {