OS X has the PAM library, but only 10.6 has the headers, so check for those. NOTE...

OS X has the PAM library, but only 10.6 has the headers, so check for those.  NOTE: a compiler check is forced because, when cross-compiling on 10.6 using the 10.5 or 10.4 SDK, the preprocessor will detect the headers in /usr/include, but the compiler can't compile them, since the SDK does not have those headers.

git-svn-id: svn://svn.code.sf.net/p/tigervnc/code/trunk@4149 3789f03b-4d11-0410-bbf8-ca57d06f2519