fix(cmake): remove extra = in flto assignment

[varunagrawal]

Description

When compiling a library with Pybind11 v2.13.1 on MacOS with Clang, CMake throws the error

clang: error: unsupported argument '' to option 'flto='

This is due to a stray = when defining flto= in the Clang case within pybind11Common.cmake.
This PR fixes this issue by removing the extra =, and after that the library compiles normally.

Suggested changelog entry:

Remove extra = when assigning flto value in the case for Clang.

[henryiii]

FYI, as a way to work around this, you can use CMake instead of our custom flags by defining CMAKE_INTERPROCEDURAL_OPTIMIZATION.

[varunagrawal]

@henryiii I saw the note about CMAKE_INTERPROCEDURAL_OPTIMIZATION in the compiling.rst doc but figured a more fundamental fix would be nice to have so things work out of the box. :)

[henryiii]

Sure, just wanted to make sure you weren't stuck until the next release. Really hoping CPython 3.13b3 comes out soon so we can move to PyMutex for 3.13 before release, but we might need the GCC 9 fix sooner.

I think this bug has been around for a while. Don't think we've touched this too much recently.

[henryiii]

Thanks, by the way, if I didn't say so before. :)

[varunagrawal]

You're very welcome. Happy to contribute fixes and reduce your burden where I can (in contrast to just raising an issue).

Thank you for the prompt response. Pybind is in good hands.