If we were conditionally enabling _BSD_SOURCE or _SVID_SOURCE, I can
understand trying to play nice with the compiler and only pass as
needed, however this is not the case and glibc <= 2.19 will not care
that we are passing _DEFAULT_SOURCE. This removes an unnecessary
build step and gives us less to maintain from a build system
perspective.
Small cosmetic changes to MacroQbtCompilerSettings are included.
Our previous setup lead to two unintended consequences:
* Debug flags were included in both release and debug builds instead
of just debug builds
* Clang doesn't support -gX (where X is the level of debugging
optimization), but we checked for -Og support instead
This commit avoids both of these scenarios by removing the additional
flags altogether. Partial resolution to #11856.
