adding check for ctags

We need ctags to run the make target 'update-map-file' so we better give
a warning out if ctags wasn't found.

Makefile.am

@@ -87,6 +87,13 @@ libcoap_@LIBCOAP_API_VERSION@_la_LDFLAGS = -version-info $(LT_LIBCOAP_CURRENT):$
 
 ## Helper target to generate the symbol table needed by libtool
 update-map-file:
+	@if [ "$(CTAGS_PROG)" = "" ]; then \
+		echo ;\
+		echo "There was no ctags program found by the configure script!" ;\
+		echo "ctags is needed for running this target! Please note the warning about the missed ctags program of the configure script." ;\
+		echo ;\
+		exit 1;\
+	fi
 	( echo "VER_$(LIBCOAP_API_VERSION) {" ; \
 	echo "global:" ; \
 	ctags -f - --c-kinds=p $(libcoap_include_HEADERS) | awk '/^coap_/ { print "  " $$1 ";" }' | sort ; \

configure.ac

@@ -91,6 +91,15 @@ AC_SUBST(LIBCOAP_API_VERSION)
 # FIXME! Switch to configure section?
 AC_PATH_PROG(DOXYGEN, doxygen, [:])
 
+# Check for ctags
+AC_ARG_VAR([CTAGS_PROG],[the 'ctag' program to use for make target 'update-map-file'])
+AC_PATH_PROG([CTAGS_PROG],[ctags],[])
+if test "x$CTAGS_PROG" = "x"; then
+	AC_MSG_NOTICE([==> Note: 'ctags' command not found!])
+	AC_MSG_WARN([==> Without ctags you will be unable to run the target 'update-map-file'!])
+	AC_MSG_WARN([==> This is no problem if you just want to build the library libcoap.])
+fi
+
 # Checks for header files.
 AC_CHECK_HEADERS([assert.h arpa/inet.h limits.h netdb.h netinet/in.h \
                   stdlib.h string.h strings.h sys/socket.h sys/time.h \