Fix backtrace call and variable setting in CMake

[AviaAv]

Description

Two likely issues in cmake/check_c_compiler_uses_glibc.cmake:

1. Type mismatch. backtrace(&buffer, size) passes void *(*)[1] where void ** is expected. On gcc >= 14 (default-error on -Wincompatible-pointer-types), the probe fails to compile on glibc. Fix: drop the &.

2. Result not propagated. set(${result_variable} ${result_variable} PARENT_SCOPE) looks like it's missing a $. In a small repro, if(USES_GLIBC) evaluates truthy regardless of probe outcome. Fix: ${${result_variable}}.

Is this user-facing behavior change?

Possibly. _GNU_SOURCE would only be defined when glibc is actually detected, and the configure log on gcc >= 14 + glibc would no longer print Performing Test USES_GLIBC - Failed.

Did you use Generative AI?

Yes. Claude Code helped diagnose and verify. I reviewed the changes myself.

Additional Information

Repro for 1: gcc -Werror=incompatible-pointer-types on the extracted snippet. Repro for 2: minimal CMakeLists.txt calling the function and printing ${USES_GLIBC}.