build: restore functionality of configure's --without-kbuild option

configure.ac

@@ -49,6 +49,7 @@ if grep -q "CentOS release 5\." /etc/redhat-release 2>/dev/null ||
 	# Well, just a warning. Maybe the admin updated the kernel.
 	echo "WARNING: This distribution's shipped kernel is not supported.";
 fi;
+if test -n "$kbuilddir"; then
 	AC_MSG_CHECKING([kernel version that we will build against])
 	krel="$(make -sC "$kbuilddir" M=. kernelrelease)";
 	kmajor="${krel%%[[^0-9]]*}";
@@ -80,6 +81,7 @@ else
 			exit 1;
 		fi;
 	fi;
+fi;
 
 AC_SUBST([regular_CPPFLAGS])
 AC_SUBST([regular_CFLAGS])

doc/changelog.txt

@@ -2,6 +2,7 @@
 HEAD
 ====
 Fixes:
+- build: restore functionality of `configure --without-kbuild`
 - xt_LOGMARK: fix detection of untracked connection for Linux >= 2.6.36