Give warnings for unknown Doxygen commands in Doxygen parser.

Silently ignoring unknown Doxygen commands is not a reasonable default
behaviour, it's simple enough to turn off the warning if the command is really
supposed to be just ignored, but it's too easy to not notice a real problem if
it isn't.

Turn WARN_DOXYGEN_UNKNOWN_COMMAND on by default and add a test to the errors
test suite checking that it is indeed given.

Examples/test-suite/errors/Makefile.in

@@ -24,12 +24,24 @@ top_builddir = @top_builddir@
 ALL_ERROR_TEST_CASES := $(patsubst %.i,%, $(notdir $(wildcard $(srcdir)/*.i)))
 CPP_ERROR_TEST_CASES := $(filter cpp_%, $(ALL_ERROR_TEST_CASES))
 C_ERROR_TEST_CASES := $(filter-out $(CPP_ERROR_TEST_CASES), $(ALL_ERROR_TEST_CASES))
+DOXYGEN_ERROR_TEST_CASES := $(filter doxygen_%, $(C_ERROR_TEST_CASES))
+C_ERROR_TEST_CASES := $(filter-out $(DOXYGEN_ERROR_TEST_CASES), $(C_ERROR_TEST_CASES))
+
+# Always use C++ for Doxygen tests, there doesn't seem to be any need to
+# distinguish between C and C++ Doxygen tests.
+DOXYGEN_ERROR_TEST_CASES := $(DOXYGEN_ERROR_TEST_CASES:=.cpptest)
 
 ERROR_TEST_CASES := $(CPP_ERROR_TEST_CASES:=.cpptest) \
-		    $(C_ERROR_TEST_CASES:=.ctest)
+		    $(C_ERROR_TEST_CASES:=.ctest) \
+		    $(DOXYGEN_ERROR_TEST_CASES)
 
 include $(srcdir)/../common.mk
 
+# This is tricky: we need to let common.mk define SWIGOPT before appending to
+# it, if we do it before including it, its defining of SWIGOPT would override
+# whatever we do here.
+$(DOXYGEN_ERROR_TEST_CASES): SWIGOPT += -doxygen
+
 # Portable dos2unix / todos for stripping CR
 TODOS        = tr -d '\r'
 #TODOS        = sed -e 's/\r$$//' # On OSX behaves as if written 's/r$$//'

Examples/test-suite/errors/doxygen_unknown_command.i

@@ -0,0 +1,6 @@
+%module xxx
+
+/**
+    There is an \unknown Doxygen comment here.
+ */
+void foo();

Examples/test-suite/errors/doxygen_unknown_command.stderr

@@ -0,0 +1 @@
+doxygen_unknown_command.i:4: Warning 720: Doxygen parser warning: unknown command "unknown".