Make it possible to build WebGL without assimp.

[msbaines]

• Speeds up WebGL build improving developer workflow.
• Speeds up WebGL startup improving user experience.

Motivation and Context

• Speeds up WebGL build improving developer workflow.
• Speeds up WebGL startup improving user experience.

How Has This Been Tested

Verified locally.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.

[mosra]

Looks good! 🎉

Side note: USE_SYSTEM_ASSIMP is also a great productivity booster for when you actually need it. Try it out (brew install assimp) and watch the improved compile times :)