DOC: install: it's DEBUG_CFLAGS, not DEBUG, which is set to -g

The INSTALL doc stated that the DEBUG variable is set to -g by default
but that's not true, it's DEBUG_CFLAGS.
diff --git a/INSTALL b/INSTALL
index 1a5af2b..83d5631 100644
--- a/INSTALL
+++ b/INSTALL
@@ -519,10 +519,10 @@
 HAProxy can compress HTTP responses to save bandwidth. Please see section 4
 about dependencies to see the available libraries and associated options.
 
-By default, the DEBUG variable is set to '-g' to enable debug symbols. It is
-not wise to disable it on uncommon systems, because it's often the only way to
-get a usable core when you need one. Otherwise, you can set DEBUG to '-s' to
-strip the binary.
+By default, the DEBUG_CFLAGS variable is set to '-g' to enable debug symbols.
+It is not wise to disable it on uncommon systems, because it's often the only
+way to get a usable core when you need one. Otherwise, you can set DEBUG to
+'-s' to strip the binary.
 
 If the ERR variable is set to any non-empty value, then -Werror will be added
 to the compiler so that any build warning will trigger an error. This is the