macro feature tests in rsync code

[John E. Malmberg]

The tests in rsync for features have been changed to testing the value 
of a macro definition instead of previously testing if the macro was 
defined.

This testing is now inconsistent with the HAVE_SOCKETPAIR macro, where 
in some places it is tested to see if the macro is defined, and other 
places to if it has a non zero value.

I would rather not globally disable the diagnostic about undefined 
macros, as it has shown in the past to help find programming problems.

-John
wb8tyw at qsl.net
Personal Opinion Only