redefined so that the dynamic plugin can replace the builtin InnoDB
in MySQL 5.1.

ha_innodb.cc, handler0alter.cc: #include "univ.i" before any other InnoDB
header files or before defining any symbols

innodb_redefine.h: New file, to contain a mapping of symbols.  The idea
is that this file will be replaced in the build process; because this
is a large file that can be generated automatically, it does not make sense
to keep it under version control.

univ.i: #include "innodb_redefine.h" and #define ha_innobase ha_innodb

Makefile.am (ha_innodb_la_CXXFLAGS): Remove -Dha_innobase=ha_innodb

NOTE: there are still some issues in the source code.  One known issue is
the #undef mutex_free in sync0sync.h, which will cause the plugin to call the
function mutex_free in the builtin InnoDB.  The preprocessor symbols defined
in innodb_redefine.h must not be undefined or redefined anywhere in the code.