diff --git a/bin/build-cmark.sh b/bin/build-cmark.sh
index cc35b6c..7917bba 100755
--- a/bin/build-cmark.sh
+++ b/bin/build-cmark.sh
@@ -3,15 +3,20 @@
 # Build the cmark-gfm library and extensions within CURRENT DIRECTORY.
 #
 # USAGE:
-#   $ build-cmark.sh [ VERSION [ TARDIR ] ]
+#   $ build-cmark.sh [ VERSION [ TARDIR [ OUTPUTDIR ] ] ]
 #
 #   VERSION: defaults to 0.28.3.gfm.12
 #   TARDIR: where to find a downloaded/cached tarball of the cmark
 #           code, or where to place a tarball
+#   OUTPUTDIR: where to create the lib/ directory. Defaults to within
+#              the top-level checkout of the tarfile.
+#              If specified, the directory must exist, and must not contain
+#              a lib directory (to ensure clean output)
 #
 
 # Echo all of our steps
 set -x
+set -e # fail on error
 
 #VERSION=0.28.3.gfm.20  ### not yet
 VERSION=0.28.3.gfm.12
@@ -21,12 +26,13 @@
 TARDIR="."
 if [ "$2" != "" ]; then TARDIR="$2"; fi
 
+# Where to put the lib directory
+# this is checked at the start of the build
+OUTPUTDIR=${3:-.}
+
 ARCHIVES="https://github.com/github/cmark/archive"
 LOCAL="${TARDIR}/cmark-gfm.$VERSION.orig.tar.gz"
 
-# WARNING: this must agree with the parent directory in the tar file or the build will fail
-EXTRACTED_AS="cmark-gfm-$VERSION"
-
 # Follow redirects, and place the result into known name $LOCAL
 if [ -f "$LOCAL" ]; then
     echo "Using cached tarball: ${LOCAL}"
@@ -35,25 +41,45 @@
     curl -L -o "$LOCAL" "$ARCHIVES/$VERSION.tar.gz" || exit 1
 fi
 
-# Clean anything old, then extract and build.
-### somebody smart could peek into the .tgz. ... MEH
-if [ -d "$EXTRACTED_AS" ]; then rm -r "$EXTRACTED_AS"; fi
-tar xzf "$LOCAL"
+# Extract into temp dir, so can detect tarfile parent directory
+TMPDIR=temp.$$
+if [ -d "$TMPDIR" ]; then rm -rf "$TMPDIR"; fi
+mkdir "$TMPDIR"
+
+# extract into clean work directory
+tar -C "$TMPDIR" -xzf "$LOCAL"
+
+# find top-level directory name (assume only one)
+EXTRACTED_AS=$(ls $TMPDIR)
+
+# clear out any old build
+if [ -d "$EXTRACTED_AS" ]; then rm -rf "$EXTRACTED_AS"; fi
+
+# move extract out of temporary work directory
+mv $TMPDIR/$EXTRACTED_AS .
+rmdir $TMPDIR
+
+# now build
 pushd "$EXTRACTED_AS"
+  # OUTPUTDIR must exist; lib must not (to avoid stale files)
+  # (do this first, to avoid later disappointment)
+  mkdir ${OUTPUTDIR}/lib # do not use -p here!
+  # get full name
+  LIBPATH=$(realpath ${OUTPUTDIR}/lib)
+
   mkdir build
   pushd build
     cmake -DCMARK_TESTS=OFF -DCMARK_STATIC=OFF ..
     make
   popd
 
-  mkdir lib
-  cp -Pp build/src/lib* lib/
-  cp -Pp build/extensions/lib* lib/
+  cp -Pp build/src/lib* ${LIBPATH}
+  cp -Pp build/extensions/lib* ${LIBPATH}
 popd
 
 # These files/dir may need a reference with LD_LIBRARY_PATH.
 # gfm.py wants this lib/ in LIBCMARKDIR.
-ls -laF "$EXTRACTED_AS/lib/"
+ls -laF "$LIBPATH"
 
 # Provide a handy line for copy/paste.
-echo "export LIBCMARKDIR='$(pwd)/$EXTRACTED_AS/lib'"
+echo "export LIBCMARKDIR='$LIBPATH'"