turn Makefile more distribution friendly

turn Makefile more distribution friendly

Changes:

 * Accept directory settings from environment.
 * Remove redundant ROOTDIR variable.
 * Set KERNEL_INCLUDE default to '/usr/include'.
 * Use CFLAGS from environemnt.

Note: In the long term it might be better to improve the configure
script to generate those parts of the Makefile in a manner similar
to autoconf. It might be even practical to autotoolize the package.

Signed-off-by: Pavel Šimerda <psimerda@redhat.com>