Respect explicit CMake IPO configuration

[sylvesterkaczmarek]

Addresses the IPO/LTO part of #392.

Summary

Release builds default CMAKE_INTERPROCEDURAL_OPTIMIZATION to ON. The current condition checks NOT CMAKE_INTERPROCEDURAL_OPTIMIZATION, which means an explicit -DCMAKE_INTERPROCEDURAL_OPTIMIZATION=OFF also enters that branch and is immediately changed back to ON.

This changes the defaulting condition to NOT DEFINED CMAKE_INTERPROCEDURAL_OPTIMIZATION.

The result is:

• Release builds still enable IPO by default when the user did not configure it.
• Explicit ON remains enabled.
• Explicit OFF is respected, allowing users to disable LTO for toolchains where it causes compiler or linker failures.

Validation

• Final diff: 1 file, +2/-1 lines, one commit, with no unrelated changes.
• Branch is based on current upstream main at ff572a21e7c2bf9fda62e1862a758da7e9a8719b.
• Full builds with the affected Ubuntu toolchains were not run locally in this environment; upstream CI should validate the CMake configuration.