BUILD: report the whole feature set with their status in haproxy -vv

It's not convenient not to know the status of default options, and
requires the user to know what option is enabled by default in each
target. With this patch, a new "Features list" line is added to the
output of "haproxy -vv" to report the whole list of known features
with their respective status. They're prefixed with a "+" when enabled
or a "-" when disabled. The "USE_" prefix is removed for clarity.
diff --git a/Makefile b/Makefile
index 14d9163..f621ca8 100644
--- a/Makefile
+++ b/Makefile
@@ -445,7 +445,8 @@
 # This variable collects all USE_* values except those set to "implicit". This
 # is used to report a list of all flags which were used to build this version.
 # Do not assign anything to it.
-BUILD_OPTIONS := $(foreach opt,$(use_opts),$(call ignore_implicit,$(opt)))
+BUILD_OPTIONS  := $(foreach opt,$(use_opts),$(call ignore_implicit,$(opt)))
+BUILD_FEATURES := $(foreach opt,$(patsubst USE_%,%,$(use_opts)),$(if $(USE_$(opt)),+$(opt),-$(opt)))
 
 ifneq ($(USE_LINUX_SPLICE),)
 OPTIONS_CFLAGS += -DCONFIG_HAP_LINUX_SPLICE
@@ -883,6 +884,7 @@
 	      -DBUILD_CC='"$(strip $(CC))"' \
 	      -DBUILD_CFLAGS='"$(strip $(VERBOSE_CFLAGS))"' \
 	      -DBUILD_OPTIONS='"$(strip $(BUILD_OPTIONS))"' \
+	      -DBUILD_FEATURES='"$(strip $(BUILD_FEATURES))"' \
 	       -c -o $@ $<
 
 install-man: