xbps-src: new vopt_feature helper

Manual.md

@@ -1204,6 +1204,11 @@ package accordingly. Additionally, the following functions are available:
   Outputs `-D<property>=true` if the option is set, or
   `-D<property>=false` otherwise.
 
+- *vopt_feature()* `vopt_feature <option> <property>`
+
+  Same as `vopt_bool`, but uses `-D<property=enabled` and
+	`-D<property>=disabled` respectively. 
+
 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

@@ -36,3 +36,11 @@ vopt_bool() {
     fi
     vopt_if "$1" "-D${prop}=true" "-D${prop}=false"
 }
+
+vopt_feature() {
+    local opt="$1" prop="${2:-$1}"
+    if [ "$#" -gt "2" ]; then
+        msg_error "vopt_feature $opt: $(($# - 2)) excess parameter(s)\n"
+    fi
+    vopt_if "$1" "-D${prop}=enabled" "-D${prop}=disabled"
+}