Simpler bootstrapping

[vanrein]

• Bootstrapping offers options from the ${CMAKE_SOURCE_DIR}/bootstrap/ directory. Collect them all. Try to find the possible Debian systems as separate options.
• Let the symlink just be one of the bootstrapping options (scan documentation for remarks about that too).
• Most of the times, a single bootstrap option is selected. This can then be automatically selected (when no setting was made yet).
• When multiple options are available, setup a bootstrap menu (including the symbolic link option). Deselect the current ${BUILDROOT_MKHERE} selection if it is not contained in the bootstrap menu.
• When no options are available, switch to manual, but then require it having been setup, or fail.
• After selection, check the presence of ${BUILDROOT_MKHERE}/lib/stdlib as a sanity check.
• Continue to work with ${BUILDROOT_MKHERE} in the files CMakeLists.txt, as we do now.