Add infrastructure changes to build the API documentation.

doxygen and graphviz are required. It's disabled by default, use
the BUILD_API_DOCS make(1) argument to enable it.

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extra : convert_revision : xtraeme%40gmail.com-20100123013719-pkk3a3823cx09v8l

include/mainpage.h

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+/**
+ * @mainpage The X Binary Package System Library API
+ * @section intro_sec Introduction
+ *
+ * XBPS is a new binary package system designed and implemented from
+ * scratch, by <b>Juan Romero Pardines</b>. This document describes
+ * the API used by the XBPS Library, that is the base to implement
+ * a package manager frontend, such as is implemented in the xbps-bin(8)
+ * and xbps-repo(8) command line interfaces.
+ *
+ * XBPS uses extensively NetBSD's proplib, a library that provides an
+ * abstract interface for creating and manipulating property lists.
+ * Property lists have object types for boolean values, opaque data, numbers,
+ * and strings. Structure is provided by the array and dictionary collection
+ * types. Property lists can be passed across protection boundaries by
+ * translating them to an external representation. This external representation
+ * is an XML document whose format is described by the following DTD:
+ *
+ * http://www.apple.com/DTDs/PropertyList-1.0.dtd
+ *
+ * NetBSD's proplib has been choosed because it's fast, extensible, and easy
+ * to use. These are the three facts I mentioned:
+ *
+ *  - <b>Fast</b> because proplib uses an ultra optimized
+ *    <em>red-black tree</em> implementation to store and find all its objects,
+ *    the same implementation has been used in commercial projects by
+ *    <em>Apple Inc</em>.
+ *
+ *  - <b>Extensible</b> because you don't have to worry about ABI problems
+ *    with its objects, arrays and dictionaries can be extended without such
+ *    problems.
+ *
+ *  - <b>Easy</b> to use (and learn) because it has a superb documentation
+ *    available in the form of manual pages.
+ *
+ * Not to mention that its arrays and dictionaries can be externalized to
+ * files (known as plists) and <b>are always written atomically</b>. You
+ * have the whole file or don't have it at all.
+ */