autoconf: Use m4sh syntax where appropriate

autoconf: Use m4sh syntax where appropriate

This fixes an issue where when using the options

  --without-embedded-libevent --without-libbsd

configure errors out with missing libevent:

  checking for libevent >= 2.0.5... no
  configure: error: *** libevent not found

but succeeds to find libevent when using

  --without-embedded-libevent --with-libbsd

configure options instead.
See also: https://autotools.info/autoconf/m4sh.html