NO_LAYOUTTESTS
Move default feature flags definition from buildsystem to the Nix code.

Buildsystem just inform the modified flags, otherwise Nix should compile
with all default features without buildsystem intervention.

This patch is based on Lazlo Gombos patch for EFL [1]

[1] https://bugs.webkit.org/show_bug.cgi?id=110717

Reviewed by: Marcelo Lira