sntp/libopts/m4/libopts.m4: allow override shell with POSIX_SHELL

Pull in fix from upstream AutoGen [1] to accept POSIX_SHELL from the
environment during the configure step.

[1] http://git.savannah.gnu.org/cgit/autogen.git/commit/?id=db064b9a252f3ef3d8db25411ea0edb0ff8ea758

Signed-off-by: James Byrne <james.byrne@origamienergy.com>

diff --git a/sntp/libopts/m4/libopts.m4 b/sntp/libopts/m4/libopts.m4
--- a/sntp/libopts/m4/libopts.m4
+++ b/sntp/libopts/m4/libopts.m4
@@ -114,6 +114,7 @@
   AC_PROG_SED
   [while :
   do
+      test -x "$POSIX_SHELL" && break
       POSIX_SHELL=`which bash`
       test -x "$POSIX_SHELL" && break
       POSIX_SHELL=`which dash`