Adapt CppUnit to our CPPUNIT_PLUGIN_EXPORT

...which mentions extern "C", so CppUnit's TestPlugInSignature must reflect
that.  It is a bit odd how CPPUNIT_PLUGIN_EXPORT needs to be passed into
CppUnit, so probably best to keep this as a local patch for now.  (Clang's
-fsanitize=undefined complained about the mismatch.)

Change-Id: Ied179a1afe82ceb04de4739c14cf8fadff31b80f

external/cppunit/CPPUNIT_PLUGIN_EXPORT.patch.0

+--- include/cppunit/plugin/TestPlugIn.h
++++ include/cppunit/plugin/TestPlugIn.h
+@@ -111,7 +111,7 @@
+ /*! \brief Type of the function exported by a plug-in.
+  * \ingroup WritingTestPlugIn
+  */
+-typedef CppUnitTestPlugIn *(*TestPlugInSignature)();
++extern "C" { typedef CppUnitTestPlugIn *(*TestPlugInSignature)(); }
+ 
+ 
+ /*! \brief Implements the function exported by the test plug-in

external/cppunit/UnpackedTarball_cppunit.mk

@@ -16,6 +16,7 @@ $(eval $(call gb_UnpackedTarball_add_patches,cppunit,\
 	external/cppunit/windows.patch \
 	external/cppunit/unix.patch \
 	external/cppunit/wundef.patch \
+	external/cppunit/CPPUNIT_PLUGIN_EXPORT.patch.0 \
 ))
 ifeq ($(DISABLE_DYNLOADING),TRUE)
 $(eval $(call gb_UnpackedTarball_add_patches,cppunit,\