Fix detection of thread-local storage capability for MSVC

[mapret]

This PR fixes an incorrect check for detecting whether MSVC supports thread-local storage or not.

The build system uses the cmake function try_compile to compile a test file which contains extern __thread int x, but this code always fails with MSVC since the correct thread-local attribute would be __declspec(thread) (Source from Microsoft). The __thread keyword is redefined for MSVC in cmake for the library, but this does not affect this call to try_compile. This then leads to the thread-local attribute being omitted for MSVC builds, which in turn leads to crashes when calling into the library from multiple threads at the same time.

I tested this change with the latest MSVC compiler by writing a small test program with calls METIS_PartGraphRecursive inside a std::for_each(std::execution::par, ...) loop. Without this change the code crashes even for just two parallel executions, with the change it runs successfully for a larger number of parallel executions.

Since the file conf/check_thread_storage.c is only used by the build system when the MSVC compiler is being used, it should be fine to change it like this. A PR with the same fix was also supplied to the METIS repository (METIS/pull/74)