Replace potentially unsupported use of multiline CMake generator expressions

[maddouri]

While investigating the project's CMake configuration for an unrelated issue, I noticed several instances of multiline generator expressions.

Based on my reading of the CMake documentation, this appears to be an unsupported form of syntax: relevant part of the generator expressions doc

This PR replaces these multiline generator expressions with equivalent forms:

• Single-line generator expressions.
• if()/endif() blocks.

I am not entirely sure why the current CMake files appear to work as they are, or why these multiline generator expressions do not seem to trigger any syntax errors. It is possible that the current behavior relies on subtle details of CMake's parser or other implementation-specific behavior. This change avoids relying on behavior that may not be guaranteed by the documented syntax.

If my interpretation of the CMake documentation is incorrect and multiline generator expressions are actually supported, please feel free to close this PR.

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