Get Fortran version correctly

cmake/external/openblas.cmake

@@ -30,8 +30,9 @@ IF(NOT ${CBLAS_FOUND})
     IF(CMAKE_COMPILER_IS_GNUCC)
         ENABLE_LANGUAGE(Fortran)
         if (NOT CMAKE_Fortran_COMPILER_VERSION)
-          # cmake version is too old, we cannot get fortran version, using CXX version instead.
-          set(CMAKE_Fortran_COMPILER_VERSION ${CMAKE_CXX_COMPILER_VERSION})
+          # cmake < 3.4 cannot get CMAKE_Fortran_COMPILER_VERSION directly.
+          execute_process(COMMAND ${CMAKE_Fortran_COMPILER} -dumpversion
+                    OUTPUT_VARIABLE CMAKE_Fortran_COMPILER_VERSION)
         endif()
         string(REGEX MATCHALL "[0-9]+" Fortran_VERSION ${CMAKE_Fortran_COMPILER_VERSION})
         list(GET Fortran_VERSION 0 Fortran_MAJOR)