Explicitly specify that Darwin, Linux and Solaris platforms have dladdr()

* include/private/gcconfig.h (HAVE_DLADDR): Define also for DARWIN
(conditionally), LINUX and SOLARIS (conditionally).
Detect dladdr() presence by configure

* configure.ac (have_dladdr): Add check for "dladdr" presence.
* configure.ac (HAVE_DLADDR): New AC define based on have_dladdr.