Add very rudimentary info on debugging with DFM.
Merge pull request #718 from cgay/docs

Add very rudimentary info on debugging with DFM.
Fix typo.
preceed -> precede
[projects] Don't need to ignore progress-callback.
[dfmc,inlining] Explain ambiguous copy down methods better.

This provides a more detailed explanation in the compiler condition
log (but not yet on the compiler's stderr stream).
[build] Allow --with-gc=malloc.

This isn't a documented option as it is just for compiler hacking
like using valgrind to check for memory errors.
Merge pull request #720 from waywardmonkeys/configure-allow-gc-malloc

[build] Allow --with-gc=malloc.
[configure] Check for both Darwin targets for Boehm dylib.

This code path isn't actually currently used on Darwin since we
use the static Boehm code path instead.
[verbose] Fix for linking output not being verbose.

When using -build from the command line, the verbose flag also
needs to be passed to the link command.
Merge pull request #719 from waywardmonkeys/explain-ambiguous-copy-downs

[dfmc,inlining] Explain ambiguous copy down methods better.
Fix error message.
[system] "Attemped" -> "Attempted"
[doc,library] Add short intro paragraph.
[doc,library] Re-organize dylan extensions info.
[doc,library] Improve linking, formatting, etc.
[doc,library] Improve linking.
[doc,library] Improve language extensions structure.
[doc,library] Fix links I broke.
[doc,library] Refer to Open Dylan, not Common Dylan when referring to the implementation.