remove optimization-inhibiting behavior from configure's --enable-debug

this behavior turned out to be counter-intuitive to users and in any
case it's unnecessary. optimization can be disabled explicitly using
the --disable-optimize option, or both can be achieved without any
enable/disable options by passing CFLAGS="-O0 -g".

configure

@@ -298,12 +298,9 @@ fi
 tryflag CFLAGS_MEMOPS -fno-tree-loop-distribute-patterns
 
 #
-# If debugging is explicitly enabled, don't auto-enable optimizations
+# Enable debugging if requessted.
 #
-if test "$debug" = yes ; then
-CFLAGS_AUTO=-g
-test "$optimize" = auto && optimize=no
-fi
+test "$debug" = yes && CFLAGS_AUTO=-g
 
 #
 # Possibly add a -O option to CFLAGS and select modules to optimize with