Build LZMA as an object, disable LZMA known warning, implement USE_WERROR and use it in Azure CI

[illwieckz]

Build LZMA as an object, disable LZMA known warning, implement USE_WERROR and use it in Azure CI.

The warning from vendored LZMA can be re-enabled with IGNORE_VENDORED_WARNING=OFF.

Include changes from:

• cmake: use -flto=auto compiler flag when supported, rework fast-math disablement #80

Because it reuses helpers from it.

[illwieckz]

MSVC has its own share of warnings, this PR only focuses on MinGW/GCC/Clang ones, so USE_WERROR isn't enabled in Appveyor CI.

[slipher]

What's the objective of making LZMA a separate library?

stringop-whatever are shit warnings that produce mostly false positives in Unvanquished too. Just turn them off everywhere.

[illwieckz]

What's the objective of making LZMA a separate library?

Actually it is already a separate library. I already moved the files to a separate folder in #76, this pushes things further.

The motivation was to be able to add specific compiler options, but it's good even without that anyway. That was just the motivation for doing it now.

stringop-whatever are shit warnings that produce mostly false positives in Unvanquished too. Just turn them off everywhere.

Uhh, ok. That can be done too.

What do you think about the general idea of using -Werror in CI? This can bite in the back when updating compilers, or updating vendored libraries.