On FreeBSD 14.0 and NetBSD 10.0, a build fails after sources have been modified:

$ touch src/cmp.c
$ ./configure
$ make V=1
...
base=`expr cmp.1 : '\(.*\).1'`				 && test -x ../src/$base					 && (echo '[NAME]'						 && sed 's@/\* *@@; s/-/\\-/;s/^GNU //; q' ../src/$base.c)	 | PATH="../src:$PATH"			 ./help2man -i - -i ./$base.x		 -S 'diffutils 2024-05-21' $base > cmp.1-t && mv cmp.1-t cmp.1
/bin/sh: ./help2man: not found
*** Error code 127

Stop.
make[1]: stopped in /home/bruno/diffutils-2024-07-21/man
*** Error code 1


The reason is that on FreeBSD, perl is at /usr/local/bin/perl, not /usr/bin/perl.
Likewise, on NetBSD, perl is at /usr/pkg/bin/perl, not /usr/bin/perl.
And likewise, on Guix, the only binary in /usr/bin is /usr/bin/env.

The attached patch fixes it. "use warnings" instead of option '-w' is
recommended per https://stackoverflow.com/questions/12554179/ ; it avoids
the use of /usr/bin/env's option '-S' which is unportable (not specified
by POSIX).