build system cleanup

Remove backup files (foo~)
Remove build artifacts (*.Po)
Remove autoconf artifacts (many)
Put a real autogen.sh in place, make it executable
Put gitignores in place to ignore these files in the future.

Tested with a clean tree, bootstrapped via autogen.sh.
Kill NO_MINUS_C_MINUS_O.  It is unused.
Make shortfin.conf sysconf_DATA
Move shortfin_startup to contrib directory

This is a very distribution-specific file; ideally it belongs with
packagers and not with an upstream project.  Move it to contrib for now
and do not install it.
Move to $(MKDIR_P) and stop hardcoding paths

Use ${localstatedir} as the location to install directories.
Organize shortfin_SOURCES a bit

Also, add header files to _SOURCES to ensure they are distributed.