Add CMake integration

[seanballais]

I am using ImPlot in a project of mine. My project uses CMake, and having CMake support in dependencies that I obtain from repositories makes it easier for me to use them in my project. As such, I have added a CMakeLists.txt to my fork of ImPlot so that I can integrate it more easily to my project. I am writing this PR in the hopes that my modifications might be helpful to other developers.

Developers can integrate ImPlot to their CMake project by adding the following lines in a CMakeLists.txt file:

include_directories(path/to/implot/)
add_subdirectory(path/to/implot/)

And then link ImPlot to their executable by doing

target_link_libraries(<target executable>
    implot
    <other libraries>)

if they want to link the static version of ImPlot, or

target_link_libraries(<target executable>
    implot_shared
    <other libraries>)

if they want to the shared version.

I have added three options in the CMakeLists.txt file that can be toggled on or off by setting their values to either ON or OFF, respectively:

• BUILD_IMPLOT_SHARED - builds a shared library version of ImPlot; turned on by default
• BUILD_IMPLOT_STATIC - builds a static library version of ImPlot; turned on by default
• BUILD_IMPLOT_DEMO - includes the demo in the library; turned off by default

I've also updated the README to include the documentation for CMake integration.

[epezent]

Hi! Thank you for submitting this PR. I'm sure some may find it useful. However, I will not be merging this yet as:

1. ImGui does not yet support CMake officially, so it feels strange for us to
2. Adding ImPlot sources manually to a target in CMake is about as much code as what's above. There's not much benefit that I can see.
3. I suspect that people using ImPlot either add its sources directly to their own, or add them to their ImGui library complication. Compiling ImPlot as a separate lib seems like overkill.

If and when ImGui provides support for CMake, I'll reopen the issue. To be clear, I use CMake religously and I don't see much benefit to supporting it until ImGui does. Follow their on-going discussion here: ocornut/imgui#1713

Also, just as an FYI to you and others: you should probably never link ImGui or ImPlot dynamically as both are quite call heavy.

[seanballais]

Hey, @epezent! Thanks for the feedback! 2 and 3 are good points.