FORTLINK: Guess of Unknown

Guess the FORTLINK API convention is `UNDERSCORE` for unknown values,
unless it's the classic (non-clang) IBM Fortran compiler, where it's
`LOWERCASE`.

Tools/CMake/AMReX_Defines.cmake

@@ -105,7 +105,13 @@ function ( set_amrex_defines )
          set(FORTLINK "UNDERSCORE")
          message(STATUS "Fortran name mangling scheme: ${FORTLINK} (lower case, append underscore)")
       else ()
-         message(AUTHOR_WARNING "Fortran to C mangling not compatible with AMReX code")
+         # now we have to guess
+         if (CMAKE_Fortran_COMPILER_ID MATCHES XL) # old IBM prior to XLClang
+             set(FORTLINK "LOWERCASE")
+         else ()
+             set(FORTLINK "UNDERSCORE")
+         endif()
+         message(AUTHOR_WARNING "Fortran to C mangling not compatible with AMReX code, assuming '${FORTLINK}'")
       endif ()
 
       add_amrex_define( BL_FORT_USE_${FORTLINK} )  # Only legacy form