[branch 1.2] Debug build broken

The debug build was broken with R5368:
//mscore.svn.sourceforge.net/viewvc/mscore?view=revision&revision=5368

In R5368 the missing -g option was added again:
//mscore.svn.sourceforge.net/viewvc/mscore?view=revision&revision=5398

-Wall -Wextra -Winvalid-pch is still missing.

I suggest to add it where the -g was added.