build: Fix typo in SE Linux library detection code

Although this only affected the static mode.

Regression introduced in commit 067295e958dcc9af87d2adfd0c697e9f87ef7b73.

debian/changelog

@@ -8,6 +8,8 @@ dpkg (1.18.11) UNRELEASED; urgency=medium
     - Fix M4sh/Autoconf coding style. Add a new section to coding-style.txt
       describing M4sh/Autoconf.
     - Disable C++ exceptions for dselect.
+    - Fix typo in SE Linux library detection code, only affecting static
+      mode (not used in Debian). Regression introduced in dpkg 1.18.8.
 
   [ Updated manpages translations ]
   * German (Helge Kreutzmann).

m4/dpkg-libs.m4

@@ -95,7 +95,7 @@ AC_DEFUN([DPKG_LIB_SELINUX], [
       AC_DEFINE([WITH_LIBSELINUX], [1],
         [Define to 1 to compile in SELinux support])
       PKG_CHECK_EXISTS([libselinux], [
-        AS_IF([test "x$with_selinux" = "xstatic"], [
+        AS_IF([test "x$with_libselinux" = "xstatic"], [
           dpkg_selinux_libs="-Wl,-Bstatic "$($PKG_CONFIG --static --libs libselinux)" -Wl,-Bdynamic"
         ], [
           dpkg_selinux_libs=$($PKG_CONFIG --libs libselinux)