Set #pragma once for build.h

Set #pragma once for build.h

The header is included all over the place and hence also multiple times
in the same translation unit which triggers -Wredundant-decls, beside
being pointless, so we just tell the pre-processor with a slightly more
modern form of a header guard that once is enough.

As a bonus, this commit keeps the 'DO NOT EDIT' remark as intended.