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
/diff/tests/guix-authenticate.sh?id=822e2173edc689a797355d9a16085d6e21bbed69'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/tests/guix-authenticate.sh'>
<input type='hidden' name='id' value='822e2173edc689a797355d9a16085d6e21bbed69'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=822e2173edc689a797355d9a16085d6e21bbed69'>root</a>/<a href='/guix/log/tests?id=822e2173edc689a797355d9a16085d6e21bbed69'>tests</a>/<a href='/guix/log/tests/guix-authenticate.sh?id=822e2173edc689a797355d9a16085d6e21bbed69'>guix-authenticate.sh</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/tests/guix-authenticate.sh?id=822e2173edc689a797355d9a16085d6e21bbed69&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>