bcfg2-selinux.spec: Fail silently when missing prerequisites

Previously, we were failing hard if build dependencies were not met.

/bin/sed: can't read /usr/share/selinux/devel/policyhelp: No such file or directory

This commit allows rpmbuild to fall through so that we can see the real
problem.

error: Failed build dependencies:
	/usr/share/selinux/devel/policyhelp is needed by bcfg2-selinux-1.3.1-1.noarch

Signed-off-by: Sol Jerome <sol.jerome@gmail.com>
Packages: don't cache package collections with no sources
FileProbes: made client probe compatible with bcfg2 1.2
POSIX: fix directory pruning
POSIX: fix docstring
Yum: prevent traceback with empty repository
1.3 Upgrade Tools: Added script to facilitate upgrading from Probe XML to database backend
doc: snapshots is deprecated, not new and shiny
tools: make migrate_info.py migrate "perms" (http://trac.mcs.anl.gov/projects/bcfg2/ticket/1150)
adding doc
doc: fixed unterminated reference
doc: fixed xmlschema extension error handling
doc: return correctly from xmlschema errors
misc: Updated server package to require pyinotify, not gamin
misc: Updated doc package to buildrequire libs needed to build full docs
Proxy: handle BadStatusLine errors gracefully
doc: encryption requires M2Crypto 0.18+
testsuite: disable travis-ci tests on py3k until python-daemon exists for py3k
fixed pylint test
testsuite: install older South for python 2.4 compat