CI: Fortran 2018 testing

This patch enables Fortran 2018 standard compliance testing.  We could
do 2023, but our current CI of choice doesn't yet have a compiler which
can do this.

The actual content of this PR is a decoupling of FCFLAGS_DEBUG and
FCFLAGS_FMS.  There is now a default FCFLAGS macro which is used by the
other two macros.  One can now optionally configure FCFLAGS_DEBUG
without worrying about the impact on FCFLAGS_FMS.

The motivation here is that we don't want to test for F2018/2023
compliance in FMS.

.github/actions/ubuntu-setup/action.yml

@@ -23,7 +23,7 @@ runs:
       run: |
         echo "::group::config.mk"
         cd .testing
-        echo "FCFLAGS_DEBUG = -g -O0 -Wextra -Wno-compare-reals -fbacktrace -ffpe-trap=invalid,zero,overflow -fcheck=bounds" >> config.mk
+        echo "FCFLAGS_DEBUG = -g -O0 -std=f2018 -Wextra -Wno-compare-reals -fbacktrace -ffpe-trap=invalid,zero,overflow -fcheck=bounds" >> config.mk
         echo "FCFLAGS_REPRO = -g -O2 -fbacktrace" >> config.mk
         echo "FCFLAGS_INIT = -finit-real=snan -finit-integer=2147483647 -finit-derived" >> config.mk
         echo "FCFLAGS_FMS = -g -fbacktrace -O0" >> config.mk

.testing/Makefile

@@ -83,19 +83,21 @@ export FMS_URL
 # TODO: This needs more automated configuration
 MPIRUN ?= mpirun
 
-# Generic compiler variables are pass through to the builds
+# Generic compiler variables are passed through to the builds
 export CC
 export MPICC
 export FC
 export MPIFC
 
 # Builds are distinguished by FCFLAGS
-FCFLAGS_DEBUG ?= -g -O0
+FCFLAGS ?= -g -O0
+
+FCFLAGS_DEBUG ?= $(FCFLAGS)
 FCFLAGS_REPRO ?= -g -O2
 FCFLAGS_OPT ?= -g -O3 -mavx -fno-omit-frame-pointer
 FCFLAGS_INIT ?=
 FCFLAGS_COVERAGE ?= -g -O0 -fbacktrace --coverage
-FCFLAGS_FMS ?= $(FCFLAGS_DEBUG)
+FCFLAGS_FMS ?= $(FCFLAGS)
 # Additional notes:
 # - These default values are simple, minimalist flags, supported by nearly all
 #   compilers, and are somewhat analogous to GFDL's DEBUG and REPRO builds.