configure.ac: Introduce --disable-ctrl

Using --disable-ctrl, one can disable the building of libosmoctrl.

The 'embedded' target will also automaticall disable ctrl.

Change-Id: I6912396338c5b23ae860fef2a55854d6df9a579d
diff --git a/src/ctrl/Makefile.am b/src/ctrl/Makefile.am
index 8bf3442..1817cac 100644
--- a/src/ctrl/Makefile.am
+++ b/src/ctrl/Makefile.am
@@ -5,6 +5,7 @@
 
 AM_CFLAGS = -Wall $(all_includes) -I$(top_srcdir)/include -I$(top_builddir)/include $(TALLOC_CFLAGS)
 
+if ENABLE_CTRL
 lib_LTLIBRARIES = libosmoctrl.la
 
 libosmoctrl_la_SOURCES = control_cmd.c control_if.c
@@ -18,3 +19,5 @@
 if ENABLE_VTY
 libosmoctrl_la_SOURCES += control_vty.c
 endif
+
+endif