[bug-diffutils] bug#72235: build failure on FreeBSD and NetBSD

[Bruno Haible]

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).

0001-build-Fix-use-of-perl-on-Guix-FreeBSD-NetBSD.patch
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