Guard against empty CMAKE_Fortran_COMPILER_ID

cmake/f_check.cmake

@@ -45,7 +45,7 @@ if (NOT ONLY_CBLAS)
 
   # TODO: detect whether underscore needed, set #defines and BU appropriately - use try_compile
   # TODO: set FEXTRALIB flags a la f_check?
-  if (NOT (${CMAKE_SYSTEM_NAME} MATCHES "Windows" AND ${CMAKE_Fortran_COMPILER_ID} MATCHES "IntelLLVM"))
+  if (NOT (${CMAKE_SYSTEM_NAME} MATCHES "Windows" AND x${CMAKE_Fortran_COMPILER_ID} MATCHES "IntelLLVM"))
   set(BU "_")
   file(APPEND ${TARGET_CONF_TEMP}
     "#define BUNDERSCORE _\n"