package/samba4: set WAF_NO_PREFORK to fix cross-compilation

package/samba4: set WAF_NO_PREFORK to fix cross-compilation

Since version 4.10.0 the waf build system does not correctly take the values
from the cross answers file when configuring for the target machine,
resulting in wrong values, e.g.:

```
Checking for rpath library support       : not found
```

Looking into sambas config.log shows that the check for rpath library
support aborts because of in internal exception.

The result is that the shared library libsmbclient.so does not have a
correct rpath section in its ELF header:

```
Library rpath: [/usr/lib]
```

This is incorrect, as libsmbclient links to secondary libraries located
in /usr/lib/samba.

When linking mpd (with libsmbclient feature enabled) against
libsmbclient, the linker does not find the secondary libraries for
libsmbclient and fails with:

```
/usr/lfs/v0/rc-buildroot-test/scripts/instance-2/output/host/opt/ext-toolchain/bin/../lib/gcc/powerpc64le-buildroot-linux-gnu/7.4.0/../../../../powerpc64le-buildroot-linux-gnu/bin/ld: warning: libcli-ldap-common-samba4.so, needed by /usr/lfs/v0/rc-buildroot-test/scripts/instance-2/output/host/usr/bin/../powerpc64le-buildroot-linux-gnu/sysroot/usr/lib/libsmbclient.so, not found (try using -rpath or -rpath-link)
[skip]
/usr/lfs/v0/rc-buildroot-test/scripts/instance-2/output/host/opt/ext-toolchain/bin/../lib/gcc/powerpc64le-buildroot-linux-gnu/7.4.0/../../../../powerpc64le-buildroot-linux-gnu/bin/ld: /usr/lfs/v0/rc-buildroot-test/scripts/instance-2/output/host/usr/bin/../powerpc64le-buildroot-linux-gnu/sysroot/usr/lib/libsamdb.so.0: undefined reference to `sec_privilege_id'
[skip]
```

The bug has been reported upstream [1]. Setting `WAF_NO_PREFORK=1` fixes
the internal exception by disabling the usage of pre-forked processes
which somehow fails in cross-compiling environment.

Now, the configuration takes the correct values from the cross answers file:

```
Checking for rpath library support       : yes
```

And leads to a correct rpath entry in the ELF header:

```
Library rpath: [/usr/lib/samba:/usr/lib]
```

And fixes the build of with libsmbclient feature enabled.

Fixes:
http://autobuild.buildroot.net/results/1678a6c5e5c9ee44b7a90d059d95c5d385d75132/
http://autobuild.buildroot.net/results/ba11b09c0d99b005c71ddd4db7fa4caa5e68af9c/

[1] https://bugzilla.samba.org/show_bug.cgi?id=13846

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>