devel/cmake: change some default CFLAGS from -O3 to -O2

[Alexey Dokuchaev]

On Thu, Feb 21, 2019 at 07:56:14PM +0100, Tobias C. Berner wrote:
> So it is something you should take upstream then :-)

I'm considering it.

> With my kde@ hat on I have not been convinced that there is any benefit
> in doing it.

On a related note, why a universal build tool such as CMake is maintained
by kde@ and kde@ folks decide what's benefitial for it and what's not?

./danfe