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
diff --git a/tests/wscript b/tests/wscript
index f5ebcc8..db65313 100644
--- a/tests/wscript
+++ b/tests/wscript
@@ -15,7 +15,7 @@
         install_path=None,
         )
 
-    if Utils.unversioned_sys_platform () == "darwin":
+    if bld.env['HAVE_OSX_SECURITY']:
         unittests.source += bld.path.ant_glob('**/*-osx.cpp')
 
     # In case we want to make it optional later