* CMakeLists: Move compilation flags into the src directory
We generally shouldn't be hijacking CMAKE_CXX_FLAGS, etc as a means to
append flags to the targets, since this adds the compilation flags to
everything, including our externals, which can result in weird issues
and makes the build hierarchy fragile.
Instead, we want to just apply these compilation flags to our targets,
and let those managing external libraries to properly specify their
compilation flags.
This also results in us not getting as many warnings, as we don't raise
the warning level on every external target.
* CMakeLists: Move off of modifying CMAKE_*-related flags
Modifying CMAKE_* related flags directly applies those changes to every
single CMake target. This includes even the targets we have in the
externals directory.
So, if we ever increased our warning levels, or enabled particular ones,
or enabled any other compilation setting, then this would apply to
externals as well, which is often not desirable.
This makes our compilation flag setup less error prone by only applying
our settings to our targets and leaving the externals alone entirely.
This also means we don't end up clobbering any provided flags on the
command line either, allowing users to specifically use the flags they
want.
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