xbps-src: new target: show-var <varname> to print the value of <varname>.

xbps-src

@@ -75,6 +75,9 @@ Targets: (only one may be specified)
  show-shlib-requires <pkgname>
    Show list of required shlibs for <pkgname>. Package must be installed into destdir.
 
+ show-var <var>
+   Prints the value of <var> if it's defined in xbps-src.
+
  zap
    Removes a masterdir but preserving ccache, distcc and host directories.
 
@@ -559,6 +562,10 @@ show-shlib-requires)
     read_pkg
 	show_pkg_shlib_requires
 	;;
+show-var)
+    eval value="\${$XBPS_TARGET_PKG}"
+    echo $value
+    ;;
 zap)
 	masterdir_zap
 	;;