Log Message:
Add DragonFly support. We need to override the libtool magic, otherwise
libltdl would be used and the startup time be in the dimensions of 10
minutes.

Fix the detection of isnan and isinf, both are type-variadic macros
under C99 and don't have to exist as external symbols. NetBSD is
protected by the compatibily symbols, but would fail the test if they
ever go away.