Makefile: clean up

Remove double -Wall.

Remove include/linux - include/ is sufficient.

Add 'obj' folder to the clean target.

Take JAVA_HOME environment variable for the systems with more
than one JDK instance installed.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Merge pull request #7 from yegorich/compile-fixes

Makefile: clean up