1) Modified abi_check rule to not write into the
   distribution directory.
2) Added the .pp files to EXTRA_DIST so that it will
   be included in the distribution

Makefile.am:
  Modified the abi_check rule to not generate the
  .out files in the distribution directory.
include/Makefile.am:
  Added the EXTRA_DIST entry for plugin.h.pp and
  mysql.h.pp so that they will be included while
  creating the distribution.