Makefile.defs: version set to 5.8.0-rc0

- entering the phase to prepare releasing next major version series
  5.8.x

src/Makefile.defs

@@ -107,7 +107,7 @@ INSTALL_FLAVOUR=$(FLAVOUR)
 VERSION = 5
 PATCHLEVEL = 8
 SUBLEVEL =  0
-EXTRAVERSION = -pre1
+EXTRAVERSION = -rc0
 
 # memory manager switcher
 # 0 - f_malloc (fast malloc)