To export symbols from the mysqld.exe, use lib.exe with /DEF, rather than
pre-link step when building mysqld.exe.

This helps to avoid relinking all plugins, if mysqld.exe was recompiled
but the list of its exports has not changed.

Also removed unnecessary DEPENDS in some ADD_CUSTOM_COMMAND (gen_lex_token,
gen_lex_hash etc). They confuse VS generator which tends to
recreate headers and do unnecessary recompilations.