samba4: remove dependency on specific C libraries

samba4 relies on the $ORIGIN feature of the dynamic linker, which used
to not be implemented in old uClibc versions. However:

 - this feature is supported by glibc
 - this feature is supported by uClibc-ng, which is the only uClibc
   version we are going to support
 - this feature is supported by musl

Consequently, we can completely remove the dependency of samba4 on
certain C libraries.

Note that despite this commit, samba4 still cannot be chosen when the
musl C library is used, because samba4 requires native RPC support,
which musl doesn't provide.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>