Enhance macos detection when use gnu functions

[monsieurchico]

On mac os, when you use gnu-sed, gnu-grep, etc.. the tool call theses functions like we were using regular mac os functions.
I think we should catch the error when it is the case and try to execute the linux equivalent after.
Moreover, macos seems not to like declare -A CUI_COLORS(.

[monsieurchico]

@geoffroy-aubry is it ok if I create a hotfix for this ?

[geoffroy-aubry]

OK for hotfix, but don't close it before an external reviewing :-)

[monsieurchico]

So I decided to base the os compatibility functions on error codes instead of os detection.
we saw that on some cases (for instance mac os + gnutls extension) the os detection is not relevant.

[geoffroy-aubry]

Travis CI build failed :-/