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.

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

README

@@ -2,13 +2,17 @@ See http://xbps.nopcode.org/ for information.
 
 To build this you'll need:
 
-- openssl (development package with static libs)
-- libarchive (development package with static libs, and all features
-  built in: acl/attr, xz, bzip2 and zlib are REQUIRED to build the
-  static binaries!)
-- proplib (development package with static lib),
-  available at http://code.google.com/p/portableproplib
-- asciidoc (to build the manpages)
+  - openssl (development package with static libs)
+  - libarchive (development package with static libs, and all features
+    built in: acl/attr, xz, bzip2 and zlib are REQUIRED to build the
+    static binaries!)
+  - proplib (development package with static lib),
+    available at http://code.google.com/p/portableproplib
+  - asciidoc (to build the manpages)
+
+Optionally to build the API documentation:
+
+  - graphviz and doxygen.
 
 By default it will be installed into /usr/local, can be changed
 by setting PREFIX and DESTDIR vars as make(1) arguments.