PATCH: fix libstdc++-v3 include checks on *bsd systems

[Gabriel Dos Reis]

Marc Espie <espie@schutzenberger.liafa.jussieu.fr> writes:

| Current snapshots won't build on many *bsd systems without the following
| patch. Mathconf.h incorrectly assumes it can include machine/endian.h
| without sys/types.h.

Hi Marc,

This patch is OK.

Patches about V3 should also be CC'd to V3 list.

-- Gaby
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