Check for dladdr(), rather than assuming we have it if we have RTLD_DEFAULT

Check for dladdr(), rather than assuming we have it if we have RTLD_DEFAULT

Unfortunately, Cygwin defines RTLD_DEFAULT (for glibc compatibility), but can't
provide dladdr(), so add a check for dladdr()

Since I don't think scons is ever used to build for Cygwin, just set HAVE_DLADDR
in SConscript, assuming that if we have RTLD_DEFAULT, we have dladdr().

Cc: Jonathan Gray <jsg@jsg.id.au>
Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Eric Anholt <eric@anholt.net>