Add --Wcast-qual to the compiler warning flags.

In the process of testing stricter compiler flags (WT-2682) we uncovered that the WT_PAGE.dsk structure is declared const, but is updated. The compiler flag that reveals the problem is: --Wcast-qual.

There is a case in reconciliation with overflow records where we update the disk image, see for more information.

It would be nice to be able to consider the WT_PAGE.dsk image as const, but that will require two fairly major changes:

• Finding an alternative solution to the overflow record problem
• Pushing the const'ness down into users of WT_PAGE.dsk