Remove the dead --disable-visiblity configure option

At the same time, document --disable-Bsymbolic.

configure.ac

@@ -224,10 +224,6 @@ AC_ARG_ENABLE(rebuilds,
               [AC_HELP_STRING([--disable-rebuilds],
                               [disable all source autogeneration rules])],,
               [enable_rebuilds=yes])
-AC_ARG_ENABLE(visibility,
-              [AC_HELP_STRING([--disable-visibility],
-                              [don't use ELF visibility attributes])],,
-              [enable_visibility=yes])
 
 if test "x$enable_threads" != "xyes"; then
   enable_threads=no
@@ -296,10 +292,6 @@ else
   fi
 fi
 
-if test "x$enable_visibility" = "xno"; then
-  GLIB_DEBUG_FLAGS="$GLIB_DEBUG_FLAGS -DDISABLE_VISIBILITY"
-fi
-
 # Ensure MSVC-compatible struct packing convention is used when
 # compiling for Win32 with gcc.
 # What flag to depends on gcc version: gcc3 uses "-mms-bitfields", while