patch 8.0.1551: on Mac 'maxmemtot' is set to a weird value

Problem:    On Mac 'maxmemtot' is set to a weird value.
Solution:   For Mac use total memory and subtract system memory. For other
            systems accept both a 32 bit and 64 bit result. (Ozaki Kiichi,
            closes #2646)
diff --git a/src/version.c b/src/version.c
index 048387b..389bbb1 100644
--- a/src/version.c
+++ b/src/version.c
@@ -779,6 +779,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1551,
+/**/
     1550,
 /**/
     1549,