build: harmonize Clang checks and update V8 warning cflags
- Set the clang variable in
config.gypi
so it depends on compiler checks made by the configure script. - Replace gyp conditions with
llvm_version
and "0.0" with conditions that use theclang
variable. - Always use
clang==1
orclang==0
in gyp conditions
This will help for https://github.com/nodejs/node/pull/52870