do not use non-standard include file "endian.h" in SGRID

[Roland Haas]

SGRID currently uses a file endian.h (or so) to detect endianess in some IO modules. This file does not exist on macOS and prevents SGRID from compiling there.

[Roland Haas]

Pull request is here: https://github.com/EinsteinToolkit/ExternalLibraries-SGRID/pull/1

[Roland Haas]

Please review.

[Roland Haas]

To be backported.

@Wolfgang Tichy already has a better (but larger) fix in master

[Roland Haas]

Better fix as git hash 93d909e "update sgrid.tar to a4fe199d, that should work on MacOS" of ExternalLibraries-SGRID

Backported as git hash 041bc1f "SGRID: detect endianness at runtime" of ExternalLibraries-SGRID

[Roland Haas]

@Steven R. Brandt