meson: Handle the b_lto option as a string option

Since 3e6fbde94c1cb8d4e01b7daf0282c315ff0e6c7d in meson (past the 0.56 release), the b_lto option was changed from a bool to a tristate option (false/true/thin).

This causes meson warnings (comparing a bool to a string) that warn that this comparison is going to be an error in the future, but this seems to be the suggested way of handling the issue:

https://github.com/mesonbuild/meson/commit/3e6fbde94c1cb8d4e01b7daf0282c315ff0e6c7d

(Not sure if we should wait for clarifications to the comments I posted on the commit, before merging this though.)