Rename build directory to obj.

Some buildsystem use build in the pathname and so we will filter out
e.g. docs generation.

.gitignore

@@ -3,6 +3,6 @@
 .*
 *.swp
 *~$
-build
+obj
 cscope.*
 tags

doc/doxy.config.in

@@ -721,7 +721,7 @@ EXCLUDE_SYMLINKS       = NO
 EXCLUDE_PATTERNS       = */.git/* \
                          */.svn/* \
                          */cmake/* \
-                         */build/*
+                         */obj/*
 
 # The EXCLUDE_SYMBOLS tag can be used to specify one or more symbol names
 # (namespaces, classes, functions, etc.) that should be excluded from the