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
c72c83ab00f9e70a264f0a638d30c02b095bdbbb'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/gnu/packages/uml.scm'>
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<div class='path'>path: <a href='/guix/log/?id=c72c83ab00f9e70a264f0a638d30c02b095bdbbb'>root</a>/<a href='/guix/log/gnu?id=c72c83ab00f9e70a264f0a638d30c02b095bdbbb'>gnu</a>/<a href='/guix/log/gnu/packages?id=c72c83ab00f9e70a264f0a638d30c02b095bdbbb'>packages</a>/<a href='/guix/log/gnu/packages/uml.scm?id=c72c83ab00f9e70a264f0a638d30c02b095bdbbb'>uml.scm</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/gnu/packages/uml.scm?id=c72c83ab00f9e70a264f0a638d30c02b095bdbbb&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-12-24 15:56:57 -0500'>2020-12-24</span></td><td><a href='/guix/commit/gnu/packages/uml.scm?id=88795880d79993e18e0ea5b2c90a3dcf7488d16b'>gnu: plantuml: Update to 1.2020.24.</a><span class='msg-avail'>...</span></td><td>Michael Rohleder</td></tr>
<tr><td><span title='2020-10-14 16:31:45 +0200'>2020-10-14</span></td><td><a href='/guix/commit/gnu/packages/uml.scm?id=2d0bea9c6d98a0dec99218935239dca8b676b86f'>gnu: plantuml: Update to 1.2020.19.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2020-09-24 09:16:50 +0200'>2020-09-24</span></td><td><a href='/guix/commit/gnu/packages/uml.scm?id=89ba5f9479a001c89872e42bd1ffdc82bf02b5f7'>gnu: plantuml: Update to 1.2020.17.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2020-09-18 11:18:09 +0200'>2020-09-18</span></td><td><a href='/guix/commit/gnu/packages/uml.scm?id=6182897c98e11676b9a370da297a688c311f4cab'>gnu: plantuml: Update to 1.2020.16.</a><span class='msg-avail'>...</span></td><td>Michael Rohleder</td></tr>
<tr><td><span title='2020-08-01 09:13:40 +0200'>2020-08-01</span></td><td><a href='/guix/commit/gnu/packages/uml.scm?id=033b43b2684334655e9c778d8a63f4f61561e759'>gnu: plantuml: Update to 1.2020.15.</a><span class='msg-avail'>...</span></td><td>Michael Rohleder</td></tr>
<tr><td><span title='2020-02-25 13:47:40 +0100'>2020-02-25</span></td><td><a href='/guix/commit/gnu/packages/uml.scm?id=1fc47a9b491bcc6d3a216592f00f71468a2c80f0'>gnu: plantuml: Update to 1.2020.1.</a><span class='msg-avail'>...</span></td><td>Jelle Licht</td></tr>
<tr><td><span title='2020-01-19 17:57:08 +0100'>2020-01-19</span></td><td><a href='/guix/commit/gnu/packages/uml.scm?id=b2e213c3e9c58dc9539a29dcb152fe20df767281'>gnu: plantuml: Use HTTPS home page.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>