Only install headers when component is enabled.

[mering]

Summary of Changes

When Qt or demos are disabled, make install should not install the respective headers. This PR takes the existing flags into account to only install these headers when the respective feature is enabled.

Release Management

• Affected package(s):
• Issue(s) solved (if any):
• Feature/Small Feature (if any):
• Link to compiled documentation:
• License and copyright ownership:

[sloriot]

Thanks for the contribution!