Makefile: add cross compile capability

Adapt the Makefile to be cross-compile-able and add a install command.
This also makes the library Yocto Project compatible.

Signed-off-by: Richard Leitner <richard.leitner@skidata.com>
Polished pull request: No need to install headerfile. Use variables for -fPIC and -shared.
Merge branch 'rleitner-fix-makefile'