mpas-model: Overwrite compiler args in Makefile (#31431)

Use 8 byte reals only when `precision=double` is set

The pre-defined compilation targets do not follow the usual behavior of
Makefiles. Compiler flags are set as strings (not Makefile variables) and as
such are not able to be overridden by environment variables. This patch changes
the behavior to the expected behavior of a Makefile such that `fflags` etc have
the desired effect.

Co-authored-by: Stephen Sachs <stesachs@amazon.com>

var/spack/repos/builtin/packages/mpas-model/package.py

@@ -5,6 +5,7 @@
 import os
 
 from spack.package import *
+from spack.util.executable import Executable
 
 
 class MpasModel(MakefilePackage):
@@ -58,6 +59,21 @@ class MpasModel(MakefilePackage):
              git='https://github.com/MPAS-Dev/MPAS-Data.git',
              tag='v7.0')
 
+    def patch_makefile(self, action, targets):
+        """Patch predefined flags in Makefile.
+        MPAS Makefile uses strings rather Makefile variables for its compiler flags.
+        This patch substitutes the strings with flags set in `target:`."""
+
+        # Target `all:` does not contain any strings with compiler flags
+        if action == "all":
+            return
+
+        sed = Executable('sed')
+        for target in targets:
+            key = target.split('=')[0]
+            sed("-i", "-e",  "/^" + action + ":.*$/,/^$/s?" + key + ".*\\\" \\\\?"
+                + target + "\\\" \\\\?1", "Makefile")
+
     def target(self, model, action):
         spec = self.spec
         satisfies = spec.satisfies
@@ -80,10 +96,12 @@ def target(self, model, action):
             ])
         elif satisfies('%intel'):
             fflags.extend([
-                '-r8',
                 '-convert big_endian',
                 '-FR',
             ])
+            if satisfies('precision=double'):
+                fflags.extend(['-r8'])
+
             cppflags.append('-DUNDERSCORE')
         targets = [
             'FC_PARALLEL={0}'.format(spec['mpi'].mpifc),
@@ -121,6 +139,7 @@ def target(self, model, action):
             'CORE={0}'.format(model), action
         ])
 
+        self.patch_makefile(action, targets)
         return targets
 
     def build(self, spec, prefix):