Allow the user to set the ctags binary

[vicentebolea]

In some distros (e.g OpenSUSE) universal ctags is installed as universal-ctags. This change allows the user to set the binary of ctags to anything using the $CTAGS env variable.

[vicentebolea]

@rurban I wonder if you could review this

[vicentebolea]

@linuxhw

[rurban]

Wrong. The my declaration must be outside the block, the logic is wrong, and a typo in ENV

[rurban]

This is enough:

my $CTAGS = $ENV{"CTAGS"} ? $ENV{"CTAGS"} : "ctags";

[vicentebolea]

@rurban thanks for the review, I applied your changes. Please find it in the last push, feel free to squash merge since the orginal commit has no value.

[vicentebolea]

@scutuatua-crypto merge me!