Remove the test for linux/if_tun.h. It has been available for years on any decent Linux distribution. Although linux/if_tun.h is now required to compile tinc, you can still run it on systems which only support Ethertap.
Remove autogen.sh, the autoreconf program does exactly that. Update everything for the latest autoconf and automake versions.
Update configure scripts.
Autoconf cleanup. Works for both 2.13 and 2.53, although running autoconf 2.53 still gives some errors.
Correct inclusion of standard if_tun.h header file.
Small fix to make it compile again
Reinstated search for if_tun.h in kernel source tree, because apparently /usr/include/linux does not always have the same contents as the include files from the currently running kernel.
tinc_TUNTAP now substitutes the values outside the AC_CACHE_CHECK block. configure should now correctly set HAVE_TUNTAP.
Get rid of the annoying empty line
Skip the check for Linux kernel sources
Really #include the if_tun.h files now
Linearized checks for if_tun.h
Add checks for the presence of the universal tun/tap device driver.