build: Correction of OSX Security Framework detection

When a non-Apple compiler is used (e.g., gcc-4.8 from macports), OSX
Security Framework cannot be used, but security framework detection was
not failing.  This commit makes security framework optional on OSX
platform, waf will present warning and `*-osx.*` files will not be
compiled in such cases.

Note that on OSX 10.9 even with this fix, compilation with gcc-4.8 will
still fail, because of STL library incompatibility (e.g., boost and
cryptopp libraries usually compiled against Apple-provided STL and
gcc-4.8 uses by default gcc's STL).

Change-Id: Iecc69f8913fa2f5fea62ac22918d6bc0f05d77c4
Refs: #1443
security: Enable load of ValidatorConf from a boost::property_tree section

The functionality was already there, but was marked private.
This commit makes this function a public interface of ValidatorConf.

Change-Id: I5de598bc43949685963bff979959775733c3a639