update mir to 2.20.2, switch to ninja, reenable lto, force o2, add mi…

[sudopluto]

• update to 2.20.2
• use ninja as build system rather than makefiles
• add bash and systemd-libs as explicit depends
• renable LTO
• force -O2 to be used for build
• make compiler warnings not fatal to build
• other MIR build variables, like the disabling of some tests that they don't support outside of Ubuntu

[FabioLolix]

Hi, this apply none or comment the previous review #54