add vopt_conflict helper

vopt_conflict prints an error and exits if two options are set at the
same time.

Manual.md

@@ -535,6 +535,10 @@ package accordingly. Additionally, the following functions are available:
   Same as `vopt_with`, but uses `--enable-<flag>` and
   `--disable-<flag>` respectively.
 
+- *vopt_conflict()* `vopt_conflict <option 1> <option 2>`
+
+  Emits an error and exits if both options are set at the same time.
+
 The following example shows how to change a source package that uses GNU
 configure to enable a new build option to support PNG images:
 

common/environment/setup/options.sh

@@ -19,3 +19,9 @@ vopt_enable() {
     vopt_if "$1" "--enable-${flag}" "--disable-${flag}"
 }
 
+vopt_conflict() {
+    local opt1="$1" opt2="$2" n1="build_option_$1" n2="build_option_$2"
+    if [ "${!n1}" -a "${!n2}" ]; then
+        msg_error "options '${opt1}' and '${opt2}' conflict\n"
+    fi
+}