Add DESTDIR support

When building rpm packages it is required to install the resulting
binaries below an "offset" directory. Such directory is specified with
make DESTDIR=$RPM_BUILD_ROOT in almost all packages.

Without this change make install will fail. Using make
SOME_DIRECTORY_VARIABLE=$RPM_BUILD_ROOT/$WHATEVER_WAS_IN_SOME_DIRECTORY_VARIABLE
will fail when SOME_DIRECTORY_VARIABLE is compiled into the binaries.
The offset will become part of the desired directories. This leads to
runtime errors. And it leads to package build failures because the
post-build checks scan for RPM_BUILD_ROOT usage in binaries.

Signed-off-by: Olaf Hering <olaf@aepfle.de>