add_library & and_executable omitted sources added back

[fyazici]

for cmake 3.10.x and below compatibility. related to issue #757 (by me) and #759 (by another user)

[bruxisma]

Hi apologies about this. I forgot to add a space between the target name and the closing parentheses. This is technically how its supposed to be done. In other words:

add_library(yaml-cpp ${yaml-cpp-type} "")

would also work. That said, before this gets merged @jbeder, either the put an empty string approach should be taken, or the target_sources calls need to be removed. I'd prefer to keep the target_sources calls in, but that's a personal preference for "readability" (even if add_library(name "") isn't as readable.

[fyazici]

@slurps-mad-rips no worries. I will try your suggestion on my 16.04 machine just in case. I am closing this PR for now. If that works as expected I will open a new PR with your solution.