Remove generated header file from the /src tree

Description

A generated header that is created in the source tree causes problems for multiple build directories using the same tree. The plugin-macros header has this problem. It could be solved by moving the generated file into the build directory, but CMake provides a better option. Instead of using a generated .h file, use the add_compile_definitions() macro to add the needed macros to the compiler configuration. This allows the generation of plugin-macros.generated.h to be dropped entirely.

How Has This Been Tested?

Tested locally on MacOS and Ubuntu, and in GitHub actions CI

Types of changes

Tweak (non-breaking change to improve existing functionality)

Checklist:

[x] My code has been run through clang-format.
[x] I have read the contributing document.
[x] My code is not on the master branch.
[x] The code has been tested.
[x] All commit messages are properly formatted and commits squashed where appropriate.
[x] I have included updates to all appropriate documentation.

obsproject / obs-plugintemplate