Fail to build on a seperate directory on Linux

[micl2e2]

During custom build from source on the root directory of shadowsocks-libev, following the instructions that current README offers is sufficient to make everything works.

But it is not true if the working directory of the building operation happens in a seperate directory(recommended by GCC, see below):

# ...... 
$ pwd
/path/to/shadowsocks-libev-SRC
$ ./autogen.sh
# ......
$ mkdir /path/to/shadowsocks-libev-BUILD
$ cd /path/to/shadowsocks-libev-BUILD
$ ../configure
$ make
# errors

In my scenarios, the errors are:

...
asciidoc: FAILED: missing configuration file: asciidoc.conf
...

Similar errors also arise for the other files resides in the doc subdirectory.

Note that the "Buiding on a Seperate Directory" is the officially recommended way to build GCC, as https://gcc.gnu.org/install/configure.html says:

First, we highly recommend that GCC be built into a separate directory from the sources......