[MEDIUM] check for cttproxy support when required

Previously, use of the "usesrc" keyword could silently fail if
either the module was not loaded, or the user did not have enough
permissions. Now the errors are better diagnosed and more appropriate
advices are given.
diff --git a/Makefile b/Makefile
index d9b12f9..a08f010 100644
--- a/Makefile
+++ b/Makefile
@@ -94,7 +94,6 @@
 # set some defines when needed.
 # Known ones are -DENABLE_POLL, -DENABLE_EPOLL, and -DUSE_MY_EPOLL
 # - use -DTPROXY to compile with transparent proxy support.
-# - use -DCONFIG_HAP_CTTPROXY to enable full transparent proxy support
 DEFINE = -DTPROXY
 
 # Now let's determine the version, sub-version and release date.
@@ -119,13 +118,16 @@
 
 # do not change this one, enable USE_* variables instead.
 OPTIONS =
+OPT_OBJS =
 
 ifneq ($(USE_TCPSPLICE),)
 OPTIONS += -DCONFIG_HAP_TCPSPLICE
 endif
 
+# - set USE_CTTPROXY to enable full transparent proxy support
 ifneq ($(USE_CTTPROXY),)
 OPTIONS += -DCONFIG_HAP_CTTPROXY
+OPT_OBJS += src/cttproxy.o
 endif
 
 ifneq ($(USE_TPROXY),)
@@ -199,7 +201,7 @@
        src/proto_http.o src/stream_sock.o src/appsession.o src/backend.o \
        src/session.o src/hdr_idx.o src/rbtree.o
 
-haproxy: $(OBJS)
+haproxy: $(OBJS) $(OPT_OBJS)
 	$(LD) $(LDFLAGS) -o $@ $^ $(LIBS)
 
 objsize: haproxy