link: fixed to include <sys/types.h> for caddr_t

When building against musl libc compiling bails out because of caddr_t not
being declared:

--------------------------------------------------------------------------
link.c: In function 'wireless_sigqual':
link.c:41:24: error: 'caddr_t' undeclared (first use in this function)
--------------------------------------------------------------------------

As caddr_t is declared in <sys/types.h> this header needs to be included.

Signed-off-by: Christian Wiese <chris@opensde.org>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
build: add DESTDIR support

This change implements support for specifying an alternative
installation destination directory by setting DESTDIR when
calling 'make install'.

Example: make DESTDIR=/alternative-rootfs install

It is particularly useful when cross-compiling and installing
in a dedicated directory where for example the rootfs or a
build sandbox for the target architecture is mounted.

By using DESTDIR you can avoid to mess with PREFIX to include
the alternative installation directory!

Signed-off-by: Christian Wiese <chris@opensde.org>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>