Don't embed the path to the build-time POSIX shell as this will be
$TMPDIR/hosttools/bash, which is no good on the target.

Instead default to /bin/sh but allow it to be set in the environment.

This isn't really upstreamable but I filed a bug at
https://gitlab.com/gnutls/gnutls/issues/807 and hope a proper fix will be
integrated.

Upstream-Status: Inappropriate
Signed-off-by: Ross Burton <ross.burton@intel.com>

diff --git a/src/libopts/m4/libopts.m4 b/src/libopts/m4/libopts.m4
index c6ad738..a62faca 100644
--- a/src/libopts/m4/libopts.m4
+++ b/src/libopts/m4/libopts.m4
@@ -112,21 +112,7 @@ AC_DEFUN([INVOKE_LIBOPTS_MACROS_FIRST],[
   AC_CHECK_FUNCS([mmap canonicalize_file_name snprintf strdup strchr \
                  strrchr strsignal fchmod fstat chmod])
   AC_PROG_SED
-  [while :
-  do
-      POSIX_SHELL=`which bash`
-      test -x "$POSIX_SHELL" && break
-      POSIX_SHELL=`which dash`
-      test -x "$POSIX_SHELL" && break
-      POSIX_SHELL=/usr/xpg4/bin/sh
-      test -x "$POSIX_SHELL" && break
-      POSIX_SHELL=`/bin/sh -c '
-          exec 2>/dev/null
-          if ! true ; then exit 1 ; fi
-          echo /bin/sh'`
-      test -x "$POSIX_SHELL" && break
-      ]AC_MSG_ERROR([cannot locate a working POSIX shell])[
-  done]
+  POSIX_SHELL="${POSIX_SHELL:-/bin/sh}"
   AC_DEFINE_UNQUOTED([POSIX_SHELL], ["${POSIX_SHELL}"],
            [define to a working POSIX compliant shell])
   AC_SUBST([POSIX_SHELL])