Modernize CMake

[Symbitic]

This pull request updates the cmake build system to use more modern features. The minimum required version is v3.12, which is found in all three GitHub runners.

Changes:

• Replaced global commands like add_compile_options with target_compile_options. CMake strongly encourages encourages this practice.
• Add a new option: CMARK_EXE. When turned off, the cmark executable will not be built. Useful for when building cmark as a subproject.
• Export cmark_static as just cmark when shared libraries are disabled.
• Update install directory for cmake files on Windows (Windows does not search lib/cmake/cmark).

[Mikenux]

Hello!

In Evince (Document Viewer), there is work in progress to support markdown files (via md -> html -> pdf conversion; see https://gitlab.gnome.org/GNOME/evince/-/merge_requests/391).

I think the build option CMARK_EXE is what is needed to only build cmark as library for a flatpak build. So, it would be nice to have this. Thanks!

[jgm]

@nwellnhof is there no build configuration currently accessible that allows building without creating the executable?

It has been a while; I don't recall all the reasons this wasn't merged, but I think they include:

• It's a very big patch, and hard for someone like me who isn't an expert on cmake to evaluate
• At the time we were considering other incompatible cmake patches
• We didn't at the time want to require cmake 3.12; in fact we've only recently increased the minimum cmake version required to 3.

I see the value in something like CMARK_EXE. Maybe it's possible to extract that part of this patch without requiring more than cmake 3.7?

[Mikenux]

@jgm: I now build with '-BUILDDIR=build' option in the json file, but '-DCMAKE_INSTALL_PREFIX=/app' does not work. However, if I edit the Makefile by removing the '?' after 'INSTALL_PREFIX' and by adding '/app' after '=', it works (I edit the file, then recompress in personal archive). Did I use the wrong option?