diff --git a/configure b/configure
index 870dd9d..701d65a 100755
--- a/configure
+++ b/configure
@@ -85,6 +85,14 @@ BUILD_DEPS=
 
 loadoptions
 
+setoption()
+{
+    local opt=$1
+    local v=$2
+    if test ! $(optexist $opt) = "1"; then error 1 "unknown option: $opt"; fi
+    eval optdisabled$opt=$v
+}
+
 for arg ; do
     case "$arg" in
         --target=*) target=${arg#*=} ;;
@@ -101,10 +109,10 @@ for arg ; do
         --prefer-static) prefer=static ;;
         --set-shared=*) eval "link${arg#*=}=1" ;;
         --set-static=*) eval "link${arg#*=}=2" ;;
-        --with-*) eval optdisabled$(expr substr "$arg" 8 ${#arg})=0 ;;
-        --without-*) eval optdisabled$(expr substr "$arg" 11 ${#arg})=1 ;;
-        --enable-*) eval optdisabled$(expr substr "$arg" 10 ${#arg})=0 ;;
-        --disable-*) eval optdisabled$(expr substr "$arg" 11 ${#arg})=1 ;;
+        --with-*)    setoption $(expr substr "$arg"  8 ${#arg}) 0 ;;
+        --without-*) setoption $(expr substr "$arg" 11 ${#arg}) 1 ;;
+        --enable-*)  setoption $(expr substr "$arg" 10 ${#arg}) 0 ;;
+        --disable-*) setoption $(expr substr "$arg" 11 ${#arg}) 1 ;;
 
         -h|--help) usage 0  ;;
         -*) error 1 "invalid option '$arg'" ;;
diff --git a/libcomain b/libcomain
index 2314c28..b5d50c4 100644
--- a/libcomain
+++ b/libcomain
@@ -172,6 +172,11 @@ loadoptions()
     return 1
 }
 
+optexist()
+{
+    if test -e meta/options/"$1"; then echo 1; else echo 0; fi
+}
+
 setneeddep()
 {
     local dep=$1