There's no point in linking to gpgme-pthread if our code isn't thread-safe...
Remove all the calls to gpgme_key_ref and our Private classes for shared_ptr,
which does the right thing. Also replace an occurrence of non-thread-safe
gpgme_strerror with it's thread-safe cousin gpgme_strerror_r. This unfortunately required to change the return type from const char* to std::string.
svn path=/branches/kdepim/enterprise4/kdepimlibs/; revision=814962