build: allow building with CMake[<3.3]

[compnerd]

The COMPILE_LANGUAGE generator expression was introduced in CMake 3.3.
Debian Jessie has an older CMake version bundled, which prevents
building on that platform without manually building CMake. This
uses a more error-prone mechanism of requiring the commonmark developer
to explicitly invoke cmark_add_compile_options on the target to ensure
that the flags are propagated for the targets without using the
generator expression. Adding the options unconditionally prevents using
CMark in a build with a non-C programming language.

Fixes: #384

[jgm]

requiring the commonmark developer to explicitly invoke cmark_add_compile_options on the target to ensure that the flags are propagated for the targets without using the generator expression

Not sure what you mean by this...do you mean that the standard build instructions won't work any more (something else needs to be done?)

[compnerd]

Not sure what you mean by this...do you mean that the standard build instructions won't work any more (something else needs to be done?)

Correct. You will find that every instance of add_executable and add_library (except the aliases) are now followed by a call to the new function. Because it is a custom function, this is something that isn't in the official CMake documentation and needs to be enforced by the project. This is something that anyone adding new libraries or binaries to this project would need to be aware of.

[jgm]

I don't think I made myself clear enough. What I'm asking about is whether the procedure for a user to build and install this library using cmake would be any different as a result of this change?

[compnerd]

Oh, from the user's perspective everything remains the same as before.

[jgm]

Thanks.