build.sh should check if $PERL5LIB has been set already

If $PERL5LIB has already been set by the user, then it should not forcefully be set it in line 293 I think.

I had some issues installing cil using ocaml 4.08.0, therefore, had to use 4.05.0, which lead to my path to be different from ~/.opam/system/lib/perl5 (system become 4.05.0 in my path) and me manually setting my own $PERL5LIB environment variable in my .bashrc to install ktc in the first place. @berezovskyi pointed out to me that the build.sh was setting the $PERL5LIB variable, so I had to edit the setting of $PERL5LIB out of build.sh, but then it worked fine.