fortran: ensure not to use [AM_]CPPFLAGS

Add missing bits from open-mpi/ompi@ab398f4

Signed-off-by: Gilles Gouaillardet <[email protected]>

Author: ggouaillardet

ompi/mpi/fortran/use-mpi-f08/profile/Makefile.am

@@ -28,6 +28,13 @@
 
 include $(top_srcdir)/Makefile.ompi-rules
 
+# Note that Automake's Fortran-buidling rules uses CPPFLAGS and
+# AM_CPPFLAGS.  This can cause weirdness (e.g.,
+# https://github.com/open-mpi/ompi/issues/7253).  Let's just zero
+# those out and rely on AM_FCFLAGS.
+CPPFLAGS =
+AM_CPPFLAGS =
+
 # This Makefile is only relevant if we're building the "use mpi_f08"
 # MPI bindings.
 if OMPI_BUILD_FORTRAN_USEMPIF08_BINDINGS