busybox: PAM support needs thread support

In commit f21b2558 ("busybox: added
linux-pam support"), we added optional support for PAM in
Busybox. However, this support requires the toolchain to have thread
support, which causes build failures with non-thread capable
toolchains.

This commit therefore enables Busybox PAM support only if the
linux-pam package is available *and the toolchain has thread support.

Fixes:

  http://autobuild.buildroot.org/results/1a3/1a380aaca9303b67cc59165d56cf12f35966fe26/



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