uClinux may not have dhfcn.h et.al.

Hi,

uClinux (which I'm treating as a linux variant) can be configured
without shared library support (no dl*() and no dlfcn.h).  The patch
below (hopefully attached):

- tweaks autoconf to define HAVE_DLFCN_H when "dlfcn.h" is present
(I've not otherwise changed that somewhat unusual logic)

- for linux, only enable DLL et.al. when "dlfcn.h" is present

- #ifdef'd code that used dlfcn.h and/or dl*() unconditionally

I've tried to be conservative here - a more aggressive version would
throw out much of the hard-wired unix dl* configuration and just rely
on autoconf.

Andrew

BTW, beyond a successful build, is there anything I should be looking
for when running "make" in pr/tests (on a native Linux system, say)?