Fix MinGW linker errors: explicitly link dbghelp on Windows (#10203)

Building Z3 on Windows with MinGW (`mingw-w64-ucrt-x86_64-gcc`) fails
with undefined references to `SymSetOptions`, `SymInitialize`,
`SymFromAddr`, and `SymGetLineFromAddr64` from the DbgHelp library used
in `src/util/debug.cpp` for stack backtraces.

The existing `#pragma comment(lib, "dbghelp.lib")` in `debug.cpp` is an
MSVC-only extension — MinGW silently ignores it, so `dbghelp` is never
passed to the linker.

## Changes

- **`CMakeLists.txt`**: Append `dbghelp` to `Z3_DEPENDENT_LIBS` in the
`WIN32` platform block. This feeds the library to both the `libz3` and
`shell` link steps regardless of compiler.
- **`src/util/debug.cpp`**: Guard the `#pragma comment` with `#ifdef
_MSC_VER` to make the MSVC-only intent explicit.

<!-- START COPILOT CODING AGENT SUFFIX -->

- Fixes #10171

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

src/util/debug.cpp

@@ -39,7 +39,9 @@ bool assertions_enabled() {
 #if defined(_WINDOWS)
 #include <windows.h>
 #include <dbghelp.h>
+#ifdef _MSC_VER
 #pragma comment(lib, "dbghelp.lib")
+#endif
 static void print_windows_backtrace() {
     HANDLE process = GetCurrentProcess();
     SymSetOptions(SYMOPT_LOAD_LINES | SYMOPT_DEFERRED_LOADS | SYMOPT_UNDNAME);