configure.ac: Fix --without-caps option

irqbalance links to libcap-ng if it is installed, even when useflag
caps is disabled. This is an error in the configure-script, which
checks for the wrong variable to determine if the user used --with/
--without for libcap-ng.

Downstream bug report: https://bugs.gentoo.org/660782

Signed-off-by: Lars Wendler <polynomial-c@gentoo.org>

configure.ac

@@ -65,7 +65,7 @@ AC_ARG_WITH([libcap-ng],
   AS_HELP_STRING([libcap-ng], [Add libcap-ng-support @<:@default=auto@:>@]))
 
 AS_IF(
-  [test "x$libcap_ng" != "xno"],
+  [test "x$with_libcap_ng" != "xno"],
   [
   PKG_CHECK_MODULES([LIBCAP_NG], [libcap-ng],
     [AC_DEFINE(HAVE_LIBCAP_NG,1,[libcap-ng support])],