remove BUILD_IOUTIL symbol

Add ioutil_stubs.c to provide an empty ioutil_init() routine.
Add ioutil.h to prevent applications from needing to declare it.

Allows unconditionally calling that function during startup, and the
resulting libocdhelper library API is now more stable.

Prints a DEBUG message when the stub implementation is included.

src/helper/Makefile.am

@@ -27,6 +27,8 @@ libhelper_la_SOURCES = \
 
 if IOUTIL
 libhelper_la_SOURCES += ioutil.c
+else
+libhelper_la_SOURCES += ioutil_stubs.c
 endif
 
 libhelper_la_CFLAGS =
@@ -38,6 +40,7 @@ endif
 noinst_HEADERS = \
 	binarybuffer.h \
 	configuration.h \
+	ioutil.h \
 	types.h \
 	log.h \
 	command.h \