Fix FTBFS on GNU/kFreeBSD (and likely Gnu/Hurd)

Fix FTBFS on GNU/kFreeBSD (and likely Gnu/Hurd)

Forwarded: yes
Origin: vendor

The original failure [1] was caused by the fact that cxxabi.h execinfo.h etc.
includes where guarded more restrictively (e.g. additionally by  __linux) than
the code which needs them (just by e.g.
KWSYS_SYSTEMINFORMATION_HAS_CPP_DEMANGLE).  kFreeGNU has some of these headers
hence extend __linux ifdef to kFreeBSD and GNU Hurd as well.

Ideally, the code should be fixed to have the same level of ifdef protection in
all places.

[1] https://buildd.debian.org/status/fetch.php?pkg=cmake&arch=kfreebsd-amd64&ver=2.8.12-1&stamp=1383507045

Gbp-Pq: Name fix-ftbfs-on-kfreebsd.patch