client VM variant build error: zgc (enabled) depends on opto (disabled)

Introduced since Dec 03, "8214476: ZGC: Build ZGC by default".

# ./configure --with-jvm-variants=client
# make CONF=linux-x86_64-normal-client-release
...
In file included from /home/build/jdk11u/src/hotspot/share/opto/regmask.hpp:29:0,
                 from /home/build/jdk11u/src/hotspot/share/opto/compile.hpp:42,
                 from /home/build/jdk11u/src/hotspot/share/opto/node.hpp:29,
                 from /home/build/jdk11u/src/hotspot/share/opto/multnode.hpp:28,
                 from /home/build/jdk11u/src/hotspot/share/opto/cfgnode.hpp:28,
                 from /home/build/jdk11u/src/hotspot/share/opto/loopnode.hpp:28,
                 from /home/build/jdk11u/src/hotspot/share/gc/shared/c2/barrierSetC2.hpp:30,
                 from /home/build/jdk11u/src/hotspot/share/gc/z/c2/zBarrierSetC2.hpp:27,
                 from /home/build/jdk11u/src/hotspot/share/gc/z/zBarrierSet.cpp:26:
/home/build/jdk11u/src/hotspot/share/opto/optoreg.hpp:32:1: fatal error: adfiles/adGlobals_x86.hpp: No such file or directory
 #include CPU_HEADER(adfiles/adGlobals)