Add the 'guild' and 'guile' variables to 'guile-2.0.pc'.

Add the 'guild' and 'guile' variables to 'guile-2.0.pc'.

* configure.ac: Remove meta/guile-2.0.pc and
  meta/guile-2.0-uninstalled.pc from 'AC_CONFIG_FILES'.
* meta/Makefile.am (substitute): New variable.
  (guile-2.0.pc, guile-2.0-uninstalled.pc): New targets.
  (guile-config, guild): Use $(substitute) instead of duplicated sed
  script.
  (CLEANFILES): Add the .pc files.
* meta/guile-2.0.pc.in (bindir, guild, guile): New variables.
* doc/ref/libguile-parallel.texi (Parallel Installations): Document the
  'guild' and 'guile' pkg-config variables.