Convert comment to compile-time check, fix grammar

By converting the text describing the constraints of the constant
to a compile-time check, this enables us to make IO_SIZE configurable.

It should be noted that this #define is duplicated in the submodule
libmariadb in the include/ma_global.h file.

Signed-off-by: Eric Herman <[email protected]>

Author: ericherman

include/my_global.h

@@ -672,12 +672,15 @@ typedef SOCKET_SIZE_TYPE size_socket;
 #endif
 
 /*
-  Io buffer size; Must be a power of 2 and a multiple of 512. May be
-  smaller what the disk page size. This influences the speed of the
-  isam btree library. eg to big to slow.
+  I/O buffer size. May be smaller than the disk page size.
+  This influences the speed of the isam btree library. E.g.: too big too slow.
   4096 is a common block size on SSDs.
 */
 #define IO_SIZE			4096U
+#if ((IO_SIZE <= 0) || ((IO_SIZE % 512) != 0) || ((IO_SIZE & (IO_SIZE-1)) != 0))
+#error "IO_SIZE must be a positive multiple of 512 and power of 2"
+#endif
+
 /*
   How much overhead does malloc have. The code often allocates
   something like 1024-MALLOC_OVERHEAD bytes