Make ecbuild_add_lang_flags more verbose about all flags used on failure

[Andrew-Beggs-ECMWF]

Is your feature request related to a problem? Please describe.

This issue has come up a few times in CloudSC where invalid flags are read from a toolchain.cmake file into CMAKE_Fortran_FLAGS. While this is the fault of the user, the first time the error is picked up is typically by ecbuild_add_fortran_flags() as CMake uses try_compile() to validate these flags. This is actually something CMake warns about on the wiki page for the underlying check_fortran_compiler_flag() that ecbuild calls.

Annoyingly this makes it seem like the flag passed to ecbuild_add_fortran_flags() is invalid, instead of the flag from the toolchain.cmake file.

Describe the solution you'd like

Could ecbuild_add_lang_flags() add variables such as CMAKE_<LANG>_FLAGS to its error message? Ideally I'd like to have the error message from the try_compile(), but this is wrapped by check_fortran_compiler_flag() and so we only get the boolean result of pass/fail. I think this might be sufficient to prompt the user to look at all flags vs just the one they were attempting to add.

I'm fine to add this change/a variation of it myself if approved.

Describe alternatives you've considered

• Error message from the compiler
  • Not feasible, see above

Additional context

No response

Organisation

ECMWF