configure: accept --with-gaproot

configure

@@ -1,17 +1,36 @@
 #!/bin/sh
 # usage: configure gappath
-# this script creates a `Makefile' from `Makefile.in' 
-if test -z $1; then 
-  GAPPATH=../..; echo "Using ../.. as default GAP path"; 
-else 
-  GAPPATH=$1; 
-fi
-if ! test -e $GAPPATH/sysinfo.gap; then
-  echo "Please give correct GAP path as argument (and make sure that GAP"
-  echo "is already properly installed)."
-  exit
+# this script creates a `Makefile' from `Makefile.in'
+
+set -e
+
+GAPPATH=../..
+while test "$#" -ge 1 ; do
+  option="$1" ; shift
+  case "$option" in
+    --with-gaproot=*) GAPPATH=${option#--with-gaproot=}; ;;
+    -*)               echo "ERROR: unsupported argument $option" ; exit 1;;
+    *)                GAPPATH="$option" ;;
+  esac
+done
+
+if test ! -r "$GAPPATH/sysinfo.gap" ; then
+    echo
+    echo "No file $GAPPATH/sysinfo.gap found."
+    echo
+    echo "Usage: ./configure [GAPPATH]"
+    echo "       where GAPPATH is a path to your GAP installation"
+    echo "       (The default for GAPPATH is \"../..\")"
+    echo
+    echo "Either your GAPPATH is incorrect or the GAP it is pointing to"
+    echo "is not properly compiled (do \"./configure && make\" there first)."
+    echo
+    echo "Aborting... No Makefile is generated."
+    echo
+    exit 1
 fi
 
-. $GAPPATH/sysinfo.gap
+echo "Using settings from $GAPPATH/sysinfo.gap"
+rm -f Makefile
+. "$GAPPATH/sysinfo.gap"
 sed -e "s|@GAPARCH@|$GAParch|g" Makefile.in > Makefile
-echo "Makefile successfully created."