glibc detection is broken (patch included)

Hi,

The test in cmake/check_c_compiler_uses_glibc.cmake doesn't work reliably because backtrace(3) is not a glibc unique feature (e.g. NetBSD libc has a compatible backtrace(3) function).

Also, the result of the test is not properly affected to the parent scope variable.

Here is a quick fix:

--- cmake/check_c_compiler_uses_glibc.cmake~	2026-05-06 20:34:17.000000000 +0200
+++ cmake/check_c_compiler_uses_glibc.cmake	2026-06-04 19:56:41.358940607 +0200
@@ -21,16 +21,15 @@
   include(CheckCSourceCompiles)
 
   set(GLIBC_TEST_CODE [====[
-    #include <execinfo.h>
+#include <features.h>
 
-    int main() {
-      void * buffer[1];
-      int size = sizeof(buffer) / sizeof(void *);
-      backtrace(&buffer, size);
-      return 0;
-    }
+#if defined(__GLIBC__)
+int main() { return 0; }
+#else
+#error
+#endif
   ]====])
 
   check_c_source_compiles("${GLIBC_TEST_CODE}" ${result_variable})
-  set(${result_variable} ${result_variable} PARENT_SCOPE)
+  set(${result_variable} ${${result_variable}} PARENT_SCOPE)
 endfunction()

Start with cmake/check_c_compiler_uses_glibc.cmake, focusing on GLIBC_TEST_CODE and how check_c_source_compiles stores and propagates its result. Compare the current detection with the included patch, then verify that the result distinguishes glibc from compatible non-glibc libc implementations and is available in the parent scope.