Build error at value_guest.c:73

Start with README.md and reproduce the reported command, ./bin/rake spec --trace, using the listed GCC environment. Inspect ext/mruby_engine/value_guest.c around line 73 and the included mruby headers; done means the engine builds past this error and the spec command completes.

Hello.
I'm trying to build engine as its described in Readme.md, but I've encountered an error:
$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/6.3.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc-multilib/src/gcc/configure --prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/ --enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++ --enable-shared --enable-threads=posix --enable-libmpx --with-system-zlib --with-isl --enable-__cxa_atexit --disable-libunwind-exceptions --enable-clocale=gnu --disable-libstdcxx-pch --disable-libssp --enable-gnu-unique-object --enable-linker-build-id --enable-lto --enable-plugin --enable-install-libiberty --with-linker-hash-style=gnu --enable-gnu-indirect-function --enable-multilib --disable-werror --enable-checking=release
Thread model: posix
gcc version 6.3.1 20170109 (GCC)

./bin/rake spec --trace
** Invoke spec (first_time)
** Invoke compile (first_time)
** Invoke compile:x86_64-linux (first_time)
** Invoke compile:mruby_engine:x86_64-linux (first_time)
** Invoke copy:mruby_engine:x86_64-linux:2.4.0 (first_time)
** Invoke lib/mruby_engine (first_time, not_needed)
** Invoke tmp/x86_64-linux/mruby_engine/2.4.0/mruby_engine.so (first_time)
** Invoke tmp/x86_64-linux/mruby_engine/2.4.0/Makefile (first_time, not_needed)
** Invoke tmp/x86_64-linux/mruby_engine/2.4.0 (first_time, not_needed)
** Invoke ext/mruby_engine/extconf.rb (first_time, not_needed)
** Invoke ext/mruby_engine/dlmalloc.c (first_time, not_needed)
** Invoke ext/mruby_engine/eval_monitored.c (first_time, not_needed)
** Invoke ext/mruby_engine/eval_unmonitored.c (first_time, not_needed)
** Invoke ext/mruby_engine/ext.c (first_time, not_needed)
** Invoke ext/mruby_engine/host.c (first_time, not_needed)
** Invoke ext/mruby_engine/libunwind_test.c (first_time, not_needed)
** Invoke ext/mruby_engine/memory_pool.c (first_time, not_needed)
** Invoke ext/mruby_engine/memory_pool_test.c (first_time, not_needed)
** Invoke ext/mruby_engine/mruby_engine.c (first_time, not_needed)
** Invoke ext/mruby_engine/platform_darwin.c (first_time, not_needed)
** Invoke ext/mruby_engine/platform_linux.c (first_time, not_needed)
** Invoke ext/mruby_engine/value_guest.c (first_time, not_needed)
** Invoke ext/mruby_engine/value_host.c (first_time, not_needed)
** Execute tmp/x86_64-linux/mruby_engine/2.4.0/mruby_engine.so
cd tmp/x86_64-linux/mruby_engine/2.4.0
/bin/make
compiling ../../../../ext/mruby_engine/value_guest.c
In file included from /usr/include/mruby/proc.h:71:0,
from ../../../../ext/mruby_engine/mruby_engine_private.h:7,
from ../../../../ext/mruby_engine/value_guest.c:1:
../../../../ext/mruby_engine/value_guest.c: In function ‘me_value_to_host_r’:
../../../../ext/mruby_engine/value_guest.c:73:38: error: dereferencing pointer to incomplete type ‘struct kh_ht’
for (int i = kh_begin(kh), f = kh_end(kh); i < f; ++i) {
^
make: *** [Makefile:242: value_guest.o] Error 1
rake aborted!
Command failed with status (2): [/bin/make...]