cmake: fix MATH_LIBRARY check on Windows MSVC

[ServeurpersoCom]

Overview

cmake : fix MATH_LIBRARY check on Windows MSVC

find_library(MATH_LIBRARY m) returns MATH_LIBRARY-NOTFOUND on MSVC, but if (DEFINED MATH_LIBRARY) is still TRUE, leading to target_link_libraries(... PRIVATE MATH_LIBRARY-NOTFOUND) and a 'cannot open m.lib' link error. Use idiomatic if (MATH_LIBRARY) which evaluates *-NOTFOUND as false.

This restores the truthy check that was in place before #22355; DEFINED is too permissive when MATH_LIBRARY is set elsewhere as *-NOTFOUND by an unrelated find_library() call earlier in configure.

Discovered while rebasing two GGML downstream projects

Refs #9908 (original old issue with the same Windows MSVC symptom)

Additional information

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[jeffbolznv]

I had a similar error but it went away with a clean tree.

[ServeurpersoCom]

I had a similar error but it went away with a clean tree.

You're right, if I delete the build folder it goes through, but doesn't the bug remain latent as soon as find_library(MATH_LIBRARY m) is called elsewhere in the configure step (downstream project, upstream submodule, or even a simple re-configure reloading the cache), since DEFINED still evaluates to TRUE with the value MATH_LIBRARY-NOTFOUND and breaks the link, whereas if (MATH_LIBRARY) correctly evaluates *-NOTFOUND as falsy per CMake's if() contract ?

[angt]

We don’t want to call find_library(MATH_LIBRARY m) anymore as it has issues when building with GGML_STATIC=ON. The idea is to use an explicit -DMATH_LIBRARY=... when required.