Allow relative path installs

I have reorganized the code to allow path-relative installs. A relative
path is used if only the last directory of a path is different from the
hardcoded binary directory. For example, /a/b/c is relative to /a/b/d,
but /a/b/c is not relative to /a/f/g.

If the install used relative paths, then the system will use the current
binary directory, strip the /bin part, and add /lib or /share as
appropriate.

I also created new get_* functions to access compiled-in paths and
adjusted if relative installs are to be used.

I cleaned up substitute_libpath_macro() code.

I will do some doc updates now for the install file.

Patch attached and applied.

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 359-1001
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