Solved linking problem by including C++ standard library '-lstdc++'. Also added a minimal working example of Fortran-C++ interfacing in dir 'mwe'.

Makefile

@@ -1,8 +1,8 @@
-CXX=icpc
-CXXFLAGS=-O0 -debug full -traceback
-FC=ifort
-FFLAGS=-O0 -debug full -traceback
-# ARCH=-xCORE-AVX2 
+CXX = icpc
+CXXFLAGS = -O0 -debug full -traceback
+FC = ifort
+FFLAGS = -O0 -debug full -traceback
+# ARCH = -xCORE-AVX2 
 
 ## Deps & objs for the C++ stuff
 cppDEPS = cppmain.cpp SM_MaponiA3.cpp SM_MaponiA3.hpp Helpers.hpp
@@ -11,6 +11,8 @@ cppOBJ = cppmain.o SM_MaponiA3.o
 ## Deps & objs for the Fortran stuff
 fDEPS = fmain.f90 SM_MaponiA3_mod.f90
 fOBJ = SM_MaponiA3.o SM_MaponiA3_mod.o fmain.o
+fLIBS = -lstdc++
+
 
 ## Compile recipes for C++ stuff
 %.o: %.cpp $(cppDEPS)
@@ -36,4 +38,4 @@ cppSherman-Morrison: $(cppOBJ)
 
 ## Linking Fortran example program calling the C++ function 'Sherman_Morrison()'
 fSherman-Morrison: $(fOBJ)
-	$(FC) $(ARCH) $(FFLAGS) -o $@ $^
+	$(FC) $(ARCH) $(FFLAGS) $(fLIBS) -o $@ $^

mwe/compile.sh

@@ -0,0 +1 @@
+icpc -c worker.cpp && ifort -c main.f90 && ifort -lstdc++ worker.o main.o -o test

mwe/main.f90

@@ -0,0 +1,12 @@
+program test
+  use iso_c_binding
+  implicit none
+
+  interface
+    subroutine hello() bind(C, name="worker")
+    end subroutine
+  end interface
+
+  call hello()
+
+end program test

mwe/worker.cpp

@@ -0,0 +1,7 @@
+#include "worker.h"
+
+void worker()
+{
+  std::cout << "Hello, World!" << std::endl;
+}
+

mwe/worker.h

@@ -0,0 +1,6 @@
+#include <iostream>
+
+extern "C"
+{
+  void worker();
+}