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

[Alexey Dokuchaev]

On Thu, Feb 21, 2019 at 01:22:48PM +0100, Tobias C. Berner wrote:
> Moin moin
>
> Why? Is there a reason you would prefer -O2?   [ Don't take this as me
> saying no to doing it, just curious about the reasons].

Because cd /usr/ports/any/port && make -V CFLAGS -V CXXFLAGS

-O2 -pipe  -fstack-protector -fno-strict-aliasing
-O2 -pipe -fstack-protector -fno-strict-aliasing

Basically, for the same reason why Fedora did that.  In most environments
-O2 used as a balanced default.  I'm curious why CMake guys opted-in for
-O3 in the first place...

./danfe