What steps will reproduce the problem?
1. build with GYP_DEFINES="enable_gtk_renderer=0"

What is the expected output?
The build should pass

What do you see instead?
It fail with:

  x86_64-pc-linux-gnu-g++ '-DOS_LINUX' '-DMOZC_ENABLE_X11_SELECTION_MONITOR=1' '-DMOZC_BUILD' '-DCHANNEL_DEV' '-DNDEBUG' '-DQT_NO_DEBUG' '-DNO_LOGGING' '-DIGNORE_HELP_FLAG' '-DI
GNORE_INVALID_FLAG' -I. -Iout_linux/Release/obj/gen 
-Iout_linux/Release/obj/gen/proto_out  -Wall -Wno-char-subscripts 
-Wno-sign-compare -Wno-deprecated-declarations -Wwrite-stri
ngs -fPIC -fno-exceptions -fmessage-length=0 -fno-strict-aliasing 
-funsigned-char -include base/namespace.h -pipe -pthread 
-fno-omit-frame-pointer -fstack-protector --param=ssp-
buffer-size=4 -pthread -I/usr/include/ibus-1.0 -I/usr/include/glib-2.0 
-I/usr/lib64/glib-2.0/include -O2 -Wno-deprecated -std=gnu++0x -MMD -MF 
out_linux/Release/.deps/out_linux/
Release/obj.target/ibus_mozc_lib/unix/ibus/surrounding_text_util.o.d.raw -O2 
-pipe -fomit-frame-pointer -march=corei7-avx -mcx16 -msahf -mno-movbe -maes 
-mpclmul -mpopcnt -mno-a
bm -mno-lwp -mno-fma -mno-fma4 -mno-xop -mno-bmi -mno-tbm -mavx -msse4.2 
-msse4.1 -mrdrnd -mf16c -mfsgsbase --param l1-cache-size=32 --param 
l1-cache-line-size=64 --param l2-cac
he-size=3072 -mtune=generic -frecord-gcc-switches -c -o 
out_linux/Release/obj.target/ibus_mozc_lib/unix/ibus/surrounding_text_util.o 
unix/ibus/surrounding_text_util.cc
unix/ibus/mozc_engine.cc: In constructor 
‘mozc::ibus::MozcEngine::MozcEngine()’:
unix/ibus/mozc_engine.cc:257:44: error: call of overloaded 
‘unique_ptr(NULL)’ is ambiguous
unix/ibus/mozc_engine.cc:257:44: note: candidates are:
In file included from 
/usr/lib/gcc/x86_64-pc-linux-gnu/4.7.3/include/g++-v4/memory:86:0,
                 from ./unix/ibus/mozc_engine.h:33,
                 from unix/ibus/mozc_engine.cc:30:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.7.3/include/g++-v4/bits/unique_ptr.h:262:7: 
note: std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) 
[with _Tp = mozc::ibus::CandidateWindowHandlerInterface; _Dp = 
std::default_delete<mozc::ibus::CandidateWindowHandlerInterface>; 
std::unique_ptr<_Tp, _Dp> = 
std::unique_ptr<mozc::ibus::CandidateWindowHandlerInterface>] <deleted>
/usr/lib/gcc/x86_64-pc-linux-gnu/4.7.3/include/g++-v4/bits/unique_ptr.h:142:7: 
note: std::unique_ptr<_Tp, _Dp>::unique_ptr(std::unique_ptr<_Tp, _Dp>&&) [with 
_Tp = mozc::ibus::CandidateWindowHandlerInterface; _Dp = 
std::default_delete<mozc::ibus::CandidateWindowHandlerInterface>; 
std::unique_ptr<_Tp, _Dp> = 
std::unique_ptr<mozc::ibus::CandidateWindowHandlerInterface>]
/usr/lib/gcc/x86_64-pc-linux-gnu/4.7.3/include/g++-v4/bits/unique_ptr.h:136:17: 
note: constexpr std::unique_ptr<_Tp, _Dp>::unique_ptr(std::nullptr_t) [with _Tp 
= mozc::ibus::CandidateWindowHandlerInterface; _Dp = 
std::default_delete<mozc::ibus::CandidateWindowHandlerInterface>; 
std::nullptr_t = std::nullptr_t]
/usr/lib/gcc/x86_64-pc-linux-gnu/4.7.3/include/g++-v4/bits/unique_ptr.h:120:7: 
note: std::unique_ptr<_Tp, _Dp>::unique_ptr(std::unique_ptr<_Tp, _Dp>::pointer) 
[with _Tp = mozc::ibus::CandidateWindowHandlerInterface; _Dp = 
std::default_delete<mozc::ibus::CandidateWindowHandlerInterface>; 
std::unique_ptr<_Tp, _Dp>::pointer = 
mozc::ibus::CandidateWindowHandlerInterface*]
unix/ibus/ibus_mozc_lib.target.mk:167: recipe for target 
'out_linux/Release/obj.target/ibus_mozc_lib/unix/ibus/mozc_engine.o' failed


What version of the product are you using? On what operating system?
mozc-1.13.1651.102 on Gentoo/Linux

Please provide any additional information below.
Please see 
https://bugs.gentoo.org/show_bug.cgi?id=512248 for complete build log

Using nullptr instead of NULL would solve the problem.