Windows: don't preset CMAKE_SYSTEM_NAME

Let it be autodiscovered instead to avoid cmake thinking it's crosscompiling builds where host = target. This resolves a cmake configure error in MSYS2 caused by CMAKE_CROSSCOMPILING checks in #4294.

cmake/32-bit-toolchain.cmake

@@ -26,7 +26,9 @@
 # STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
 # THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-set (CMAKE_SYSTEM_NAME Windows)
+if (NOT CMAKE_HOST_WIN32)
+  set (CMAKE_SYSTEM_NAME Windows)
+endif()
 
 set (GCC_PREFIX i686-w64-mingw32)
 set (CMAKE_C_COMPILER ${GCC_PREFIX}-gcc)

cmake/64-bit-toolchain.cmake

@@ -26,7 +26,9 @@
 # STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
 # THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-set (CMAKE_SYSTEM_NAME Windows)
+if (NOT CMAKE_HOST_WIN32)
+  set (CMAKE_SYSTEM_NAME Windows)
+endif()
 
 set (GCC_PREFIX x86_64-w64-mingw32)
 set (CMAKE_C_COMPILER ${GCC_PREFIX}-gcc)