git compiling: Documentation/git-add.xml does not validate

Question

When compiling git I have these errors:

make[2]: Leaving directory `/home/xxx/git-master'
    XMLTO git-add.1
xmlto: /home/xxx/git-master/Documentation/git-add.xml does not validate (status 3)
xmlto: Fix document syntax or use --skip-validation option
I/O error : Attempt to load network entity http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd
/home/xxx/git-master/Documentation/git-add.xml:2: warning: failed to load external entity "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
D DocBook XML V4.5//EN" "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
I/O error : Attempt to load network entity http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd
warning: failed to load external entity "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
validity error : Could not load the external subset "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
Document /home/xxx/git-master/Documentation/git-add.xml does not validate
make[1]: *** [git-add.1] Error 13
make[1]: Leaving directory `/home/xxx/git-master/Documentation'
make: *** [doc] Error 2

What is the main problem?

Answer 1

On OSX (Mountain Lion) I had to do this:

brew install asciidoc
brew install xmlto
brew install docbook   

# then (as prompted by brew...)
#
# If you intend to process AsciiDoc files through an XML stage
# (such as a2x for manpage generation) you need to add something
# like:
#
export XML_CATALOG_FILES=/usr/local/etc/xml/catalog
#
# to your shell rc file so that xmllint can find AsciiDoc's
# catalog files.

brew install docbook-xsl

(Thanks to Nathan for providing the necessary hints). #6chars

Answer 2

Late to the party, but on cygwin the package you need for this to validate is docbook-xml45 (as implied by the DTD URI, docbook/xml/4.5/docbookx.dtd)