round() is not portable. Use rint().

src/backend/access/gist/gistbuild.c

@@ -14,6 +14,8 @@
  */
 #include "postgres.h"
 
+#include <math.h>
+
 #include "access/genam.h"
 #include "access/gist_private.h"
 #include "catalog/index.h"
@@ -420,7 +422,7 @@ calculatePagesPerBuffer(GISTBuildState *buildstate, int levelStep)
 	 */
 	pagesPerBuffer = 2 * pow(avgIndexTuplesPerPage, levelStep);
 
-	return round(pagesPerBuffer);
+	return (int) rint(pagesPerBuffer);
 }
 
 /*