Use the current shell instead of trying to find another one and
failing to do so.

diff --git a/configure b/configure
index f1adb919..7891b4dc 100755
--- a/configure
+++ b/configure
@@ -26,28 +26,6 @@
 # OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-#
-# The #!/bin/sh might be some legacy piece of crap,
-# not even up to 1990 POSIX.2 spec. So the first step
-# is to look for a better shell in some known places
-# and re-execute ourselves with that interpreter.
-#
-
-if test x$txr_shell = x ; then
-  for shell in /bin/bash /usr/bin/bash /usr/xpg4/bin/sh ; do
-    if test -x $shell ; then
-       txr_shell=$shell
-       break
-    fi
-  done
-  if test x$txr_shell = x ; then
-    echo "No known POSIX shell found: falling back on /bin/sh, which may not work"
-    txr_shell=/bin/sh
-  fi
-  export txr_shell
-  exec $txr_shell $0 ${@+"$@"}
-fi
-
 set -u
 
 #
@@ -190,13 +168,6 @@ while [ $# -gt 0 ] ; do
     exit 1
   fi
 
-  eval "var_exists=\${$var+y}"
-
-  if [ "$var_exists" != y ] ; then
-    printf "$0: nonexistent option: '%s'\n" "$1"
-    exit 1
-  fi
-
   eval "$var='$val'"
 
   eval "var_given_exists=\${${var}_given+y}"
@@ -208,6 +179,8 @@ while [ $# -gt 0 ] ; do
   shift
 done
 
+txr_shell=$CONFIG_SHELL
+
 #
 # If --help was given (or --help=<nonempty> or help=<nonempty>) then
 # print help and exit. The termination status is failed, to indicate