COMP: include /usr/local in search for system headers/libraries

- this is consistent with compiler defaults, and helps Darwin users

wmake/scripts/have_mgridgen

@@ -91,12 +91,23 @@ have_mgridgen()
         )
     elif isSystem "$MGRIDGEN_ARCH_PATH"
     then
-        header=$(findFirstFile /usr/include/$header)
+        header=$(findFirstFile /usr/local/include/$header /usr/include/$header)
 
-        library=$(findFirstFile \
-            /usr/lib/$library \
-            /usr/lib$WM_COMPILER_LIB_ARCH/$library \
-        )
+        case "$header" in
+        /usr/local/*)
+            library=$(findFirstFile \
+                /usr/local/lib/$library \
+                /usr/local/lib$WM_COMPILER_LIB_ARCH/$library \
+            )
+            ;;
+
+        *)
+            library=$(findFirstFile \
+                /usr/lib/$library \
+                /usr/lib$WM_COMPILER_LIB_ARCH/$library \
+            )
+            ;;
+        esac
     else
         unset header library
     fi