An autoconf-generated configure script doesn't enable compiler
optimization flags if CFLAGS / CXXFLAGS options are set explicitly.
We started setting CFLAGS / CXXFLAGS in commit e6abe1c9
("cmake: add extra security compiler options"). As a result, users
started experiencing performance degradation issues, like the one
described in tarantool/tarantool-ee#440.

Let's set -O2 in CFLAGS / CXXFLAGS explicitly to fix that.

Closes #8606
Needed for tarantool/tarantool-ee#440

NO_DOC=build
NO_TEST=build