Add --disable-syslog configure option

[FreziDok]

Implements --disable-syslog configuration option, as discussed in #1610

Addresses #1610

[hallyn]

Thanks. As I don't regularly use autoconf, I tested by creating a minimal configure.ac:

serge@andre:~/autoconf-t2$ cat configure.ac
AC_INIT([test], [1.0])
# Your snippet here:
AC_ARG_ENABLE([syslog],
  [AS_HELP_STRING([--disable-syslog], [disable logging via syslog() calls])],
  [
    AS_CASE([${enableval}],
        [yes|no],[],
        [AC_MSG_ERROR([bad parameter value for --enable-syslog=${enableval}])]
    )
  ],
  [enable_syslog="yes"]
)
AS_VAR_IF([enable_syslog],["yes"],[AC_DEFINE([USE_SYSLOG], [1], [Define to use syslog])])
AC_OUTPUT
AC_MSG_NOTICE([shadow ${PACKAGE_VERSION} has been configured with the following features:
        enable syslog:                  $enable_syslog
])
serge@andre:~/autoconf-t2$ autoconf
serge@andre:~/autoconf-t2$ ./configure
configure: creating ./config.status
configure: shadow 1.0 has been configured with the following features:
        enable syslog:                  yes

serge@andre:~/autoconf-t2$ ./configure --enable-syslog=yes
configure: creating ./config.status
configure: shadow 1.0 has been configured with the following features:
        enable syslog:                  yes

serge@andre:~/autoconf-t2$ ./configure --enable-syslog=no
configure: creating ./config.status
configure: shadow 1.0 has been configured with the following features:
        enable syslog:                  no

serge@andre:~/autoconf-t2$ ./configure --enable-syslog=xxx
configure: error: bad parameter value for --enable-syslog=xxx

Looks good, thanks.

[hallyn]

(Please feel free to merge after tests pass. I would enable auto-merge, but that button's not showing up... 🤷)

[alejandro-colomar]

I'll merge, from what you've said. I haven't reviewed myself. I assume everyone's happy with the code. If anyone disagrees, feel free to complain.