RFC: enable link time optimization

Fedora, SUSE or the Firefox builds provided by Mozilla use LTO by default and the numbers look promising. LTO aims to produce smaller and faster binaries.

I suggest to try LTO as a default for Alpine.

Further reading:

• https://fedoraproject.org/wiki/LTOByDefault
• https://www.ucw.cz/~hubicka/slides/opensuse2018-e.pdf
• https://hubicka.blogspot.com/2016/03/building-libreoffice-with-gcc-6-and-lto.html#more
• https://glandium.org/blog/?p=3888
• https://wiki.ubuntu.com/ToolChain/LTO