libnettle and libgmp is used by default. Old implementation uses
libgcrypt as default. If more and more Linux distributions offer
libnettle linked GnuTLS, then this change decrease library dependency
against libgcrypt and libgpg-error. The current library detection for
libnettle and libgmp is very simple, just use AC_SEARCH_LIBS.
util::generateRandomData() was written using SimpleRandomizer, thus no
external library dependency from this function.