Reimplement C++ support in ESM Makefiles

[matthewhoffman]

Merge commit 1b3de97f inadvertently removed support for C++ compilers in the ESM Makefiles. This merge reinserts the lines that were removed. It has been branched from the commit that introduced the bug (1b3de97f).

[matthewhoffman]

@mgduda , this is the fix for the bug we were seeing in E3SM using the release branch. When cice/develop was merged into develop, support for C++ compilers was accidentally lost in the ESM Makefiles. Here is the diff that was included in the merge commit:

diff --git a/src/Makefile.in.ACME b/src/Makefile.in.ACME
index ad507cf3a..7e78ac649 100644
--- a/src/Makefile.in.ACME
+++ b/src/Makefile.in.ACME
@@ -41,7 +41,6 @@ RM = rm -f
 CPP = cpp -P -traditional
 FC=$(MPIFC)
 CC=$(MPICC)
-CXX=$(MPICXX)
 NETCDF=$(NETCDF_PATH)
 PNETCDF=$(PNETCDF_PATH)
 PIO=$(INSTALL_SHAREDPATH)/pio
@@ -71,7 +70,7 @@ all:
        @echo $(CPPINCLUDES)
        @echo $(FCINCLUDES)
        ( $(MAKE) mpas RM="$(RM)" CPP="$(CPP)" NETCDF="$(NETCDF)" PNETCDF="$(PNETCDF)" \
-         PIO="$(PIO)" FC="$(FC)" CC="$(CC)" CXX="$(CXX)" SFC="$(SFC)" SCC="$(SCC)" \
+         PIO="$(PIO)" FC="$(FC)" CC="$(CC)" SFC="$(SFC)" SCC="$(SCC)" \
          CPPFLAGS="$(CPPFLAGS)" CPPINCLUDES="$(CPPINCLUDES)" FCINCLUDES="$(FCINCLUDES)" \
          FFLAGS="$(FFLAGS)" CFLAGS="$(CFLAGS)" LDFLAGS="$(LDFLAGS)" )

@@ -83,7 +82,7 @@ mpas: externals frame ops dycore drver
        ar ru lib$(COMPONENT).a $(DRIVER)/*.o

 externals:
-       ( cd external; $(MAKE) FC="$(FC)" SFC="$(SFC)" CC="$(CC)" CXX="$(CXX)" SCC="$(SCC)" FFLAGS="$(FFLAGS)" CFLAGS="$(CFLAGS)" CPP="$(CPP)" NETCDF="$(NETCDF)" CORE="$(CORE)" ezxml-lib )
+       ( cd external; $(MAKE) FC="$(FC)" SFC="$(SFC)" CC="$(CC)" SCC="$(SCC)" FFLAGS="$(FFLAGS)" CFLAGS="$(CFLAGS)" CPP="$(CPP)" NETCDF="$(NETCDF)" CORE="$(CORE)" ezxml-lib )

 drver: externals frame ops dycore
        ( cd $(DRIVER); $(MAKE) CPPFLAGS="$(CPPFLAGS)" CPPINCLUDES="$(CPPINCLUDES)" FREEFLAGS="$(FREEFLAGS)" all )

The fix was very straightforward - just undo the diff above. I'm assigning you because it touches both the ACME and CESM versions. I confirmed the fix eliminates the bug in E3SM.

[matthewhoffman]

See #558 for the original introduction of this code. Still not sure how it got removed.

[mgduda]

This looks good to me.