Static constructors for built-in providers.

Add config.h defines for each built provider and if that provider is built as a DSO. Make
changes to init functions to match: remove ctor on fi_ini and call explicitly, only have the
ctor attribute defined on the providers if built as DSO, explicitly call built-in providers
and declare their init defs in fi.h

This should work for all our supported build configurations, and should not work for static-build plus dl-loading built-in providers (which I assume is unsupported).

Tested with dso providers and built-in, needs testing for static linking and other more providers. @sayantansur @jsquyres

I can centralize a lot of the sprinkled ifdefs in one header (for example: include/provider.h) that is included by fabric.c and the various providers if that is wanted.

fixes #293 

Signed-off-by: pmmccorm <patrick.m.mccormick@intel.com>
The _D macros are never used and can be removed.  The _IF and _DF macros are only used within the elif cases and only twice.  Those defines can be removed as well.  The _C macro name is too cryptic.  I don't like having the extra define (versus using just _INI) but I don't see a clean way to eliminate it.  But at least rename the macro.  Maybe _INIT?