Building w/ Intel oneAPI LLVM compilers might require to explicitly set the C language version

[thgeorgiou]

Describe the bug
This is related to the recently merged #1893. Trying to build WRF w/ Intel's new oneAPI compilers produces many instances of the following error on my system:

gribgetbds.c:388:5: error: call to undeclared function 'grib_unthin'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]

From the manpages it seems that ICX defaults to a more recent standard of C:

Default for Linux: c++17 The compiler conforms to the 2017 ISO C++ standard. This is true also for SYCL programs.

I am not sure if something has changed between the current version of ICX and the one tested in #1893 but being explicit about using C89 fixes the issue. This needs a small change in arch/configure.defaults:

SFC             =       ifx
SCC             =       icx -std=gnu89
CCOMP           =       icx -std=gnu89
DM_FC           =       mpif90
DM_CC           =       mpicc -std=gnu89

Using -std=c89 doesn't seem to work since it doesn't define __USE_MISC and it fails to build external/RSL_LITE/.

I suspect this is similar to what happened in #1823 and led @mgs-52 to make these changes.

To Reproduce

1. Grab a fresh copy of WRF's source (v4.5), including the change from Intel oneAPI LLVM Compilers Configuration #1893.
2. Check ICX version:

❯ icx --version
Intel(R) oneAPI DPC++/C++ Compiler 2023.2.0 (2023.2.0.20230622)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /opt/intel/oneapi/compiler/2023.2.0/linux/bin-llvm
Configuration file: /opt/intel/oneapi/compiler/2023.2.0/linux/bin-llvm/../bin/icx.cfg

3. Build dependencies (OpenMPI, HDF5, netCDF4) with the same stack. No issues encountered here.
4. Configure WRF w/ Intel oneAPI and run build script:

./configure # select 78
./compile -j 1 em_real &> compile.out

Expected behavior
A successful build of WRF without changing configure.wrf.

Attachments
I am attaching a successful build log (w/ -std=gnu89) and a failed one (w/out -std=gnu89).

compile_failed.out.txt
compile_successful.out.txt

Additional context
I am not entirely sure if there is something else that is causing this issue on my system. If you think any other information and/or tests are useful I will be happy to help. Similarly, if you think this workaround is correct and it is convenient for you, I can draft a PR for it.

[weiwangncar]

@islas Can you comment on this? Thanks!

[islas]

@thgeorgiou We are in fact using an older version (2022.1) of ICX which is probably why this didn't show up. I'll go ahead and test this on a system that I can get the latest compilers on

[islas]

Testing with the newer ifx compiler version does result in the same errors. Though, I went ahead and tested out #1823 as a solution to this and with a minor edit, that does resolve this issue. I would rather we pull those changes in to address new compiler warnings rather than fall back to older standards.

[islas]

Closing as #1823 has been merged into release-v4.5.2