Add a bit about EXTRA_PATCHES.

[wblock]

en_US.ISO8859-1/books/porters-handbook/slow-porting/chapter.xml

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Spelling: s/Conditionnally/Conditionally/

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Some ports need patches that are only applied for
specific &os; versions or when a particular option is enabled
or disabled.

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Conditional patches are specified by placing the full paths to
the patch files in <varname>EXTRA_PATCHES</varname>.</para>.

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No period at end of title.

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Typo: s/Optionnally/Optionally/

No period at end of title.

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Maybe end this sentence with a specific:

..._EXTRA_PATCHES_OFF</varname> to make the patch conditional on that option.

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No blank line here.