perl scripts: remove #!/usr/bin/perl in favor of more portable... Rather than putting #!/usr/bin/perl on the first line, start with a variant of what's recommended by "man perlrun" that invokes the first "perl" program from your shell's search path. * build-aux/gitlog-to-changelog: Replace #!... as above. Add a "Local Variables" perl mode setting. Prompted by a patch from Ludovic Courtès. Improved by Eric Blake. * build-aux/useless-if-before-free: Likewise. * build-aux/announce-gen: Likewise. * build-aux/update-copyright: Likewise.