build: print the error result when the tests fail

Any diagnostic information is useful when things fail.  In my case, it
printed out the fact that the tests were segfaulting.
build: allow skipping of the hash check in the download step

This is useful when trying to bisect an issue in a dependency, and we
want to pull from a specific commit but don't want to have to download
and determine the sha1sum and md5sum of the tarball.
build: add failure message along with number in GenerateHelptags.cmake
Merge pull request #1544 from jszakmeister/run-tests-error-code

build: print the error result when the tests fail
build: probe the compiler to see if we need to turn down _FORTIFY_SOURCE

Some builds don't use Release (such as Gentoo, though it can be told
to).  So let's go a bit further and probe the compiler to see if we need
to turn down the _FORTIFY_SOURCE level.
Fix outdated msgpack-rpc doc.

Needed after neovim/python-client#58.
Merge pull request #1514 from jszakmeister/better-fortify-detection

build: probe the compiler to see if we need to turn down _FORTIFY_SOURCE
Improve Python host messages.
Merge pull request #1559 from fwalch/python-messages

Improve Python host messages.
Merge pull request #1554 from jszakmeister/allow-skipping-of-sum

build: allow skipping of the hash check in the download step
Merge pull request #1556 from elmart/fix-msgpack-rpc-docs

Fix outdated msgpack-rpc doc.
Remove NeXTSTEP references

Remove trailing whitespace as well.
Remove Atari MiNT references
Remove OS/2 references

Paul Slootman was removed from the top of os_unix.c as OS/2 is no longer
supported, but is still credited in runtime/doc/intro.txt.