Add OPENSIM_DISABLE_STATIC_TYPE_REGISTRATION cmake option

[adamkewley]

This is a patch I've been applying downstream in opensim-creator that I figured might be useful for opensim-core.

This adds an OPENSIM_DISABLE_STATIC_TYPE_REGISTRATION cmake option that can be toggled ON to build OpenSim without the static-init-time instantiator classes.

The motivation for doing this is:

• It lets downstream applications/libraries decide if/when to register OpenSim types. There are several reasons why downstream might want this:
  • The application wants to set up logging, log sinks, etc. before registration, so that all logged registration errors appear in a runtime-defined location (e.g. a UI).
  • The application wants to lazily-load parts of OpenSim
  • A bug has appeared in the application and it would be useful to debug step registration, skip registering something, etc. without having to recompile OpenSim.
• It lets downstream application/libraries decide load-order. The reason you'd want to do this is:
  • OpenSim is being loaded as one library/application, so the instantiators cannot lean on the fact that the dynamic loader is loading libraries in use-order. Right now, the only reason the static initializers work is because osimActuators.dll depends on osimCommon.dll, so the dynamic loader just happens to load one before the other - this may fail if they are merged into one binary. It may also fail if (e.g.) the log is statically initialized after a static registration function that writes to the log (a segfault I encountered with OpenSim Creator).

Brief summary of changes

• Added OPENSIM_DISABLE_STATIC_TYPE_REGISTRATION to CMakeLists.txt
• Used the resulting compile define to control whether the static initializer is emitted or not

Testing I've completed

• Similar pattern works on OpenSim Creator, which has its own application log (shown to the user via a UI) and statically compiles OpenSim into its executable.

Looking for feedback on...

CHANGELOG.md (choose one)

• updated TODO

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