m4_esyscmd_s only exists in Autoconf 2.64 or newer

m4_esyscmd_s only exists in Autoconf 2.64 or newer

Since we like to be able to keep building on RHEL6, which
has autoconf 2.61, use the older m4_esyscmd macro and strip
the newline in the version ourself.

Resolves #3471

Fixes the following error:
$ autoreconf -vif
autoreconf: Entering directory `.'
autoreconf: configure.ac: not using Gettext
autoreconf: running: aclocal --force -I m4
autoreconf: configure.ac: tracing
autoreconf: running: libtoolize --copy --force
libtoolize: putting auxiliary files in AC_CONFIG_AUX_DIR, `build-aux'.
libtoolize: copying file `build-aux/ltmain.sh'
libtoolize: putting macros in AC_CONFIG_MACRO_DIR, `m4'.
libtoolize: copying file `m4/libtool.m4'
libtoolize: copying file `m4/ltoptions.m4'
libtoolize: copying file `m4/ltsugar.m4'
libtoolize: copying file `m4/ltversion.m4'
libtoolize: copying file `m4/lt~obsolete.m4'
autoreconf: running: /usr/bin/autoconf --force
configure.ac:3: error: possibly undefined macro: m4_esyscmd_s
      If this token and others are legitimate, please use m4_pattern_allow.
      See the Autoconf documentation.
autoreconf: /usr/bin/autoconf failed with exit status: 1