.bashrc: Only set GREP_COLOR env-var on BSD-type platforms

.bashrc: Only set GREP_COLOR env-var on BSD-type platforms

Older grep versions used the GREP_COLOR env-var for colorized output
configuration. Newer GNU grep versions use the GREP_COLORS env-var,
which supports additional colorize options.  GNU grep 3.8+ will now log
a warning message if the GREP_COLOR env-var is set, warning that env-var
is deprecated. But BSD grep still relies on GREP_COLOR.

Conditionalize the setting of GREP_COLOR vs GREP_COLORS based on OS-type
[e.g. GNU grep vs BSD grep].