Fix srcdir != builddir, and add idiom for multi-output commands

Fix srcdir != builddir, and add idiom for multi-output commands

* docs.make (monoapi.zip): Handle the case where 'monoapi.tree'
exists but 'monoapi.zip' doesn't.  IOW, we should rebuild
monoapi.zip with 'rm monoapi.zip; make'.
(mono-tools.zip, mono-file-formats.zip): Likewise.
* Makefile.am (monoapi.zip, mono-tools.zip, mono-fileformats.zip): Likewise.
(monoapi.tree): Don't use $< in an explicit rule.
(mono-tools.tree, mono-file-formats.tree): Likewise.

svn path=/trunk/mono/; revision=119361