perl scripts: remove #!/usr/bin/perl in favor of more portable...

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.