Build: Header-file related cleanup

• Move headers generated by configure to include/
• Re-instate an "external" header for use by out-of-tree projects
• Include all (but only) relevant defines in the external header Ie. directory locations, and user/group names/ids but not which functions, headers or libraries are installed
• Do not include internal headers in external ones Doing so prevents out-of-tree builds
• Do not re-define variables in Makefiles (ie. -DSOMEVAR=@SOMEVAR@) This is what config.h is for
• Do not create multiple names for the same #define
• Give portability.h a more appropriate name
• Document the header files managed by configure and how they are created
• Dont define variables that dont exist to be be other variables that also dont exist
• Dont expose internal build options (like if we use alphasort for sorting)
• Tweak the names of some #defines for clarity and to avoid namespace polution
• Remove dead options

Mercurial revision: 1ab86ce14cb44024535be048b372ca90f087eed3