Fix broken functional test.

In Lua, all math is floating point.  We need to coerce the result of a
division into a integer with the `{get,set}_height` and
`{get,set}_width` window_spec functional tests.
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.
Add missing refcount increment for systemlist()

- get_system_output_as_rettv() was missing a refcount increment when
  returning an empty list, i.e. when there was no output
- we now use rettv_list_aloc() instead of list_alloc()
- issue #1530
Merge pull request #1552 from equalsraf/tb-fix-1530

add missing refcount increment for systemlist()
Merge pull request #1543 from jszakmeister/fix-broken-functional-tests

Fix broken functional test.
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.
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.