Makefile.am: look for ioctl definitions in the kernel build tree by default

Makefile.am: look for ioctl definitions in the kernel build tree by default

While most of ioctl-related kernel headers are now exported by kernel's
headers_install, some are still modules_install only.  The kernel's
headers installed into /usr/include/ are usually headers_install'ed and
therefore don't contain some internal headers we need.  The solution is
to look for modules_install'ed headers for the running kernel, and fall
back to old behavior if they aren't found.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>