Apply warning and error compile flags only to 1P code

[tlively]

Move all the configuration of error and warning flags to after we have
added the third_party subdirectory and declared all of the 3P targets.
This ensures that only the basic configuration that e.g. affects ABI is
applied to the 3P code. This will make it easier to add 3P code that
does not compile cleanly with all the warnings and errors we enable for
our own code.

[tlively]

@kripken, this uses the architecture you suggested in #7235.

[tlively]

Right, so it doesn't appear that this actually works. CMAKE_CXX_FLAGS includes all the expected flags when we descend into the 1P source subdirectories, but it seems that the point at which its value matters is when the target is defined by add_library or add_executable. The tests are failing here because -DBUILD_LLVM_DWARF is not being passed when compiling the libbinaryen sources because the binaryen target was declared before all the 1P flags were added to CMAKE_CXX_FLAGS.

We have a few options:

• Go with Apply custom compilation flags only to 1P code #7235 instead, since it uses target_compile_options and doesn't have this "time of use" problem.
• Revert Simplify cmake by having a single binaryen library target #7238 and go back to declaring separate targets for each subdirectory so they pick up the correct flags.
• Try to preserve the simplifications from Simplify cmake by having a single binaryen library target #7238 as much as possible by creating just one addition binaryen_1p target to pick up all the new flags. I'll see if I can get this working now.

[tlively]

Ok, this is vastly simplified now. The fact that CMAKE_CXX_FLAGS takes effect at the moment a target is declared made it obvious that we just needed to set all the warning and error flags after visiting third_party and before visiting our own code.

[kripken]

Nice!

(cmake... i'll never understand it)