autoconf: Improve help text for demos and DRI drivers

Changed the text for the default demos to be "auto" instead of "yes"
since that didn't make much sense in this context. Added an example
value for the --with-dri-drivers option as suggested by Brian.
diff --git a/configure.ac b/configure.ac
index b040b4d..f2f2d75 100644
--- a/configure.ac
+++ b/configure.ac
@@ -233,7 +233,7 @@
 AC_ARG_WITH(demos,
     [AS_HELP_STRING([--with-demos@<:@=DIRS...@:>@],
         [optional comma delimited demo directories to build
-        @<:@default=yes if source available@:>@])],
+        @<:@default=auto if source available@:>@])],
     with_demos="$withval",
     with_demos="$default_demos")
 if test "x$with_demos" = x; then
@@ -372,7 +372,7 @@
 dnl Which drivers to build - default is chosen by platform
 AC_ARG_WITH(dri-drivers,
     [AS_HELP_STRING([--with-dri-drivers@<:@=DIRS...@:>@],
-        [comma delimited DRI drivers to build @<:@default=auto by platform@:>@])],
+        [comma delimited DRI drivers, e.g. "i965,radeon,nouveau" @<:@default=auto@:>@])],
     with_dri_drivers="$withval",
     with_dri_drivers=yes)
 if test "x$with_dri_drivers" = x; then