Do not output unwelcome ANSI color codes

[edgar-bonet]

ANSI color escape codes are usually a nice touch when they land on a tty, as they enhance the text readability. However, when the output is being redirected, they are most often an annoyance. Even on a tty, they are sometimes not welcome, e.g. when keeping a typescript of terminal session.

This pull request makes simavr refrain from using ANSI escape codes in the following cases:

• when libsimavr is being used outside of run_avr
• when the standard output of run_avr is not a tty
• when the environment variable NO_COLOR is set to a non-empty value

The latter is a common convention, widely adopted by many libraries and programs dealing with terminal output.

This pull request is built on top of (i.e. is a superset of) #556.

Fixes: #555.

[edgar-bonet]

I had a CI fail on error: 'font_color' defined but not used. Force-pushed with the following fixup:

diff --git a/simavr/sim/run_avr.c b/simavr/sim/run_avr.c
index eab651c..e4da1e4 100644
--- a/simavr/sim/run_avr.c
+++ b/simavr/sim/run_avr.c
@@ -34,12 +34,14 @@
 
 #include "sim_core_decl.h"
 
+#ifndef NO_COLOR
 /* ANSI escape codes to be used if color is enabled. */
 static const struct text_colors font_color = {
        .green  = "\e[32m",
        .red    = "\e[31m",
        .normal = "\e[0m"
 };
+#endif
 
 static void
 display_usage(

[gatk555]

My personal view is that the first and last commits are enough. Changing the default behaviour may annoy someone.

[edgar-bonet]

@gatk555: Thanks for reviewing this and my previous PR!

Changing the default behaviour may annoy someone.

OK. I rewrote the branch keeping only the first and last commits. This required inverting the logic of the test in the last commit: instead of “if colors are welcome, enable them”, it is now “if colors are unwelcome, disable them”.