Optimize debug builds when -Og is available

Enables optimizing with -Og if GCC has the option available, this
should produce faster debug binaries without compromising debugability.

Is a privateConfig to limit it to the default Qt build.

Includes two fixes for false positives of maybe_uninitialized triggered
by -Og on gcc 4.9.

Change-Id: I466d7a4070295714189024369312e6cbd36cfacf
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>