-[ --enable-bootstrap Enable bootstrapping [no]],,
-enable_bootstrap=no)
-if test -d ${srcdir}/gcc; then
- case "$host:$target:$enable_bootstrap" in
- $build:$build:yes | *:no) ;;
- *:yes) AC_MSG_ERROR([cannot bootstrap a cross-compiler]) ;;
- *) AC_MSG_ERROR([invalid option for --enable-bootstrap]) ;;
- esac
-else
- if test $enable_bootstrap = yes; then
- AC_MSG_ERROR([cannot bootstrap without a compiler])
- fi
-fi
+[ --enable-bootstrap[=lean] Enable bootstrapping [no]],,
+enable_bootstrap=default)
+
+# Issue errors and warnings for invalid/strange bootstrap combinations.
+case "$configdirs" in
+ *gcc*) have_compiler=yes ;;
+ *) have_compiler=no ;;
+esac
+
+case "$have_compiler:$host:$target:$enable_bootstrap" in
+ *:*:*:no) ;;
+
+ # Default behavior. (We'll) enable bootstrap if we have a compiler
+ # and we are in a native configuration.
+ yes:$build:$build:default)
+ # This will become 'yes'
+ enable_bootstrap=no ;;
+
+ *:*:*:default)
+ enable_bootstrap=no ;;
+
+ # We have a compiler and we are in a native configuration, bootstrap is ok
+ yes:$build:$build:yes | yes:$build:$build:lean)
+ ;;
+
+ # Other configurations, but we have a compiler. Assume the user knows
+ # what he's doing.
+ yes:*:*:yes | yes:*:*:lean)
+ AC_MSG_WARN([trying to bootstrap a cross compiler])
+ ;;
+
+ # No compiler: if they passed --enable-bootstrap explicitly, fail
+ no:*:*:yes | no:*:*:lean)
+ AC_MSG_ERROR([cannot bootstrap without a compiler]) ;;
+
+ # Fail if wrong command line
+ *)
+ AC_MSG_ERROR([invalid option for --enable-bootstrap])
+ ;;
+esac