Darwin gfortranclang: Use C++ linker even for C programs

This is needed to access functions from the C++ standard library;
otherwise, we get link errors when building with
ESMF_SHARED_LIB_BUILD=OFF. We could alternatively specify -lstdc++, and
that seems to solve the problem in at least some cases, but this may
lead to problems such as those with exception handling that we
encountered when linking with gfortran, which led us to use the C++
linker when linking Fortran programs. So this use of the C++ linker when
linking C programs is analogous to the use of the C++ linker when
linking Fortran programs with this compiler combination.

See #112 for details.

build_config/Darwin.gfortranclang.default/build_rules.mk

@@ -10,6 +10,7 @@ ESMF_F90DEFAULT         = gfortran
 ESMF_F90LINKERDEFAULT   = clang++
 ESMF_CXXDEFAULT         = clang++
 ESMF_CDEFAULT           = clang
+ESMF_CLINKERDEFAULT     = clang++
 ESMF_CPPDEFAULT         = clang -E -P -x c
 
 ESMF_CXXCOMPILEOPTS    += -x c++ -mmacosx-version-min=10.7 -stdlib=libc++
@@ -37,6 +38,7 @@ ESMF_F90DEFAULT         = mpif90
 ESMF_F90LINKERDEFAULT   = mpiCC
 ESMF_CXXDEFAULT         = mpiCC
 ESMF_CDEFAULT           = mpicc
+ESMF_CLINKERDEFAULT     = mpiCC
 ESMF_MPIRUNDEFAULT      = mpirun $(ESMF_MPILAUNCHOPTIONS)
 ESMF_F90COMPILECPPFLAGS+= -DESMF_NO_MPI3
 ESMF_CXXCOMPILECPPFLAGS+= -DESMF_NO_MPI3
@@ -47,6 +49,7 @@ ESMF_F90DEFAULT         = mpif90
 ESMF_F90LINKERDEFAULT   = mpicxx
 ESMF_CXXDEFAULT         = mpicxx
 ESMF_CDEFAULT           = mpicc
+ESMF_CLINKERDEFAULT     = mpicxx
 ESMF_MPIRUNDEFAULT      = mpirun $(ESMF_MPILAUNCHOPTIONS)
 ESMF_MPIMPMDRUNDEFAULT  = mpiexec $(ESMF_MPILAUNCHOPTIONS)
 ESMF_F90COMPILECPPFLAGS+= -DESMF_NO_MPI3
@@ -58,6 +61,7 @@ ESMF_F90DEFAULT         = mpif90
 ESMF_F90LINKERDEFAULT   = mpicxx
 ESMF_CXXDEFAULT         = mpicxx
 ESMF_CDEFAULT           = mpicc
+ESMF_CLINKERDEFAULT     = mpicxx
 ESMF_CXXLINKLIBS       += $(shell $(ESMF_DIR)/scripts/libs.mpich3f90)
 ESMF_F90LINKLIBS       += $(shell $(ESMF_DIR)/scripts/libs.mpich3f90)
 ESMF_MPIRUNDEFAULT      = mpirun $(ESMF_MPILAUNCHOPTIONS)
@@ -69,6 +73,7 @@ ESMF_F90DEFAULT         = mpif90
 ESMF_F90LINKERDEFAULT   = mpicxx
 ESMF_CXXDEFAULT         = mpicxx
 ESMF_CDEFAULT           = mpicc
+ESMF_CLINKERDEFAULT     = mpicxx
 ESMF_MPIRUNDEFAULT      = mpirun $(ESMF_MPILAUNCHOPTIONS)
 ESMF_MPIMPMDRUNDEFAULT  = mpiexec $(ESMF_MPILAUNCHOPTIONS)
 else
@@ -79,6 +84,7 @@ ESMF_F90DEFAULT         = mpif77
 ESMF_F90LINKERDEFAULT   = mpic++
 ESMF_CXXDEFAULT         = mpic++
 ESMF_CDEFAULT           = mpicc
+ESMF_CLINKERDEFAULT     = mpic++
 ESMF_MPIRUNDEFAULT      = mpirun $(ESMF_MPILAUNCHOPTIONS)
 ESMF_MPIMPMDRUNDEFAULT  = mpiexec $(ESMF_MPILAUNCHOPTIONS)
 ESMF_F90COMPILECPPFLAGS+= -DESMF_NO_MPI3
@@ -96,6 +102,7 @@ ESMF_CXXCOMPILECPPFLAGS+= -DESMF_NO_SIGUSR2
 ESMF_F90LINKLIBS       += $(shell $(ESMF_DIR)/scripts/libs.openmpif90_forcxx $(ESMF_F90DEFAULT))
 ESMF_CXXDEFAULT         = mpicxx
 ESMF_CDEFAULT           = mpicc
+ESMF_CLINKERDEFAULT     = mpicxx
 ESMF_MPIRUNDEFAULT      = mpirun $(ESMF_MPILAUNCHOPTIONS)
 ESMF_MPIMPMDRUNDEFAULT  = mpiexec $(ESMF_MPILAUNCHOPTIONS)
 else