Define EXEC_BACKEND in pg_config_manual.h

Define EXEC_BACKEND in pg_config_manual.h

It was for unclear reasons defined in a separate location, which makes
it more cumbersome to override for testing, and it also did not have
any prominent documentation.  Move to pg_config_manual.h, where
similar things are already collected.

The previous definition on the command-line had the effect of defining
it to the value 1, but now that we don't need that anymore we just
define it to empty, to simplify manual editing a bit.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/b7053ba8-b008-5335-31de-2fe4fe41ef0f%402ndquadrant.com