Now that we do not target BeOS and also do not include the main headers
directory when building "build" binaries, we can drop the separate
config_build directory and thus also the separate BeBuild.h, and just
..-include the regular one.
The build BeBuild.h defined empty _IMPEXP_ROOT and _IMPEXP_BE preprocessor
macros that the regular one does not; so I also re-synchronized
headers which used these as needed.
82bffcc3a7