meson: Check GNU sed's availability

As explained in the comment in build-aux/Makefile.in, the
version of sed included in the FreeBSD base system is not GNU
sed, which our syntax-check rules expect; as a result, many
checks will fail with

  gmake: gsed: No such file or directory
  /bin/sh: gsed: not found

Similarly to what we're already doing with GNU make and GNU
grep, look for GNU sed during the configuration step and fail
early if it's not available.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>

build-aux/meson.build

@@ -12,8 +12,10 @@ syntax_check_conf.set('PYTHON3', python3_prog.path())
 
 if host_machine.system() == 'freebsd'
   make_prog = find_program('gmake')
+  sed_prog = find_program('gsed')
 else
   make_prog = find_program('make')
+  sed_prog = find_program('sed')
 endif
 
 if host_machine.system() == 'freebsd'
@@ -34,6 +36,7 @@ else
 endif
 
 syntax_check_conf.set('GREP', grep_prog.path())
+syntax_check_conf.set('SED', sed_prog.path())
 
 configure_file(
   input: 'Makefile.in',