Allow using configuration value in variable expansions.

Using configurations actually seems to work correctly in the "gnu" backend, so
enable using them and update the documentation to reflect it.
Use configuration-dependent default compilation flags.

Use flags appropriate for debugging when config=Debug is specified on make
command line and flags for an optimized build when config=Release is.

However, for consistency with the typical makefiles, don't use any special
flag values at all if no configuration is specified.
Accept lower case configuration names in "gnu" toolset.

For convenience and more Unix-ish feel, accept "make config=debug" as a
synonym for "make config=Debug".