Fix MinGW/Clang 64 bits detection.

CMAKE_COMPILER_IS_GNUCC is only valid for GCC.

cmake/system_check.cmake

@@ -20,11 +20,11 @@ endif()
 
 
 
-if(CMAKE_COMPILER_IS_GNUCC AND WIN32)
+if(MINGW)
     execute_process(COMMAND ${CMAKE_C_COMPILER} -dumpmachine
-              OUTPUT_VARIABLE OPENBLAS_GCC_TARGET_MACHINE
+              OUTPUT_VARIABLE OPENBLAS_MINGW_TARGET_MACHINE
               OUTPUT_STRIP_TRAILING_WHITESPACE)
-    if(OPENBLAS_GCC_TARGET_MACHINE MATCHES "amd64|x86_64|AMD64")
+    if(OPENBLAS_MINGW_TARGET_MACHINE MATCHES "amd64|x86_64|AMD64")
       set(MINGW64 1)
     endif()
 endif()