If stdout is a TTY (meaning that the output is not redirected), tell
the build tool (GNU Make or Ninja) to enable colorized output.
GNU Make and Ninja also check if their stdout is redirected and
strip color escape sequences in that case. CLICOLOR_FORCE environment
variable overrides this behavior.
With this change, if the compiler was launched with the
-fcolor-diagnostics flag and idf.py output is not redirected, the
final output in the terminal will be colorized.
(-fcolor-diagnostics is handled at CMake level by the previous commit)
d04b554065