[FFmpeg-user] error: 'x265_api_get' undeclared (first use in this function)

[lx]

hi all:
      I compile FFmpeg which support x265 by these steps:

################################################################
sudo apt-get install mercurial cmake cmake-curses-gui build-essential
# Note: if the packaged yasm is older than 1.2, you must download yasm-1.2
and build it

hg clone https://bitbucket.org/multicoreware/x265
cd x265
hg checkout 0.8
cd x265/build/linux
./make-Makefiles.bash
make
sudo make install
sudo ldconfig
###############################################################

then I configure FFmpeg
###############################################################
./configure --enable-libx264 --enable-libx265 --enable-gpl
###############################################################

This step fail.and the error messages is:
###############################################################
require_pkg_config libx265 x265 x265.h x265_api_get
check_pkg_config libx265 x265 x265.h x265_api_get
test_pkg_config libx265 x265 x265.h x265_api_get
pkg-config --exists --print-errors x265
check_func_headers x265.h x265_api_get -I/usr/local/include
-L/usr/local/lib -lx265
test_ld cc -I/usr/local/include -L/usr/local/lib -lx265
test_cc -I/usr/local/include -L/usr/local/lib
BEGIN /tmp/ffconf.6WfbgmBw/test.c
    1 #include <x265.h>
    2 #include <stdint.h>
    3 long check_x265_api_get(void) { return (long) x265_api_get; }
    4 int main(void) { int ret = 0;
    5 ret |= ((intptr_t)check_x265_api_get) & 0xFFFF;
    6 return ret; }
END /tmp/ffconf.6WfbgmBw/test.c
gcc -D_ISOC99_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE
-D_POSIX_C_SOURCE=200112 -D_XOPEN_SOURCE=600 -DPIC -std=c11
-fomit-frame-pointer -fPIC -pthread -I/usr/local/include
-I/usr/local/include -L/usr/local/lib -c -o /tmp/ffconf.6WfbgmBw/test.o
/tmp/ffconf.6WfbgmBw/test.c
/tmp/ffconf.6WfbgmBw/test.c: In function 'check_x265_api_get':
/tmp/ffconf.6WfbgmBw/test.c:3:47: *error: 'x265_api_get' undeclared (first
use in this function)*
 long check_x265_api_get(void) { return (long) x265_api_get; }
                                               ^
/tmp/ffconf.6WfbgmBw/test.c:3:47: note: each undeclared identifier is
reported only once for each function it appears in
*ERROR: x265 not found using pkg-config*
###############################################################
These messages is from ffbuild/config.log

This is why? How to solve it?
Thank you.

btw:
FFmpeg version: 4.0
ubuntu version:16.04