This complicates builds, is less compatible with Eigen3 and doesn't
give a noticable performance improvement if turned off.

This option is disabled by default.

By default we build the whole Calligra optimized for the host architecture.
When the option is on, the hottest parts of calligra will compile optimized
for several most popular architectures. The rest of the code will not use
any brand-new instructions for not breaking binary compatibility among cpus.

Short manual:
1) If you build Calligra for yourself and are not going to copy Krita binary
   to another CPU, disable this option.
2) If you build a Calligra package and are going to distribute it among users,
   then enable the option.

It gives 1.58...1.74 times better result of the composition
on Sandy Bridge. Other architectures are to be tested.