build: add requirement for libpam to configure_ac

Add requirement for libpam in the configure_ac script, such that the
build will when not present.

Tested: Verified LIBS variable holds -lpam.
Change-Id: Idad73d89bd97c38fe739e16e5e23193e47945e50
Signed-off-by: Patrick Venture <venture@google.com>
2 files changed