/usr/ports/ports-mgmt/pkg/work/pkg-1.2.2/libpkg/../external/libyaml/src/parser.c: In function 'yaml_parser_parse_flow_mapping_key':
/usr/ports/ports-mgmt/pkg/work/pkg-1.2.2/libpkg/../external/libyaml/src/parser.c:1107:17: error: variable 'dummy_mark' set but not used [-Werror=unused-but-set-variable]
     yaml_mark_t dummy_mark;     /* Used to eliminate a compiler warning. */
                 ^
/usr/ports/ports-mgmt/pkg/work/pkg-1.2.2/libpkg/../external/libyaml/src/api.c: In function 'yaml_document_delete':
/usr/ports/ports-mgmt/pkg/work/pkg-1.2.2/libpkg/../external/libyaml/src/api.c:1121:7: error: variable 'context' set but not used [-Werror=unused-but-set-variable]
     } context;
       ^
cc1: all warnings being treated as errors
*** [/usr/ports/ports-mgmt/pkg/work/pkg-1.2.2/libpkg/../external/libyaml/src/parser.o] Error code 1
cc1: all warnings being treated as errors
*** [/usr/ports/ports-mgmt/pkg/work/pkg-1.2.2/libpkg/../external/libyaml/src/api.o] Error code 1
2 errors
*** [all] Error code 2
1 error
===> Compilation failed unexpectedly.
Try to set MAKE_JOBS_UNSAFE=yes and rebuild before reporting the failure to
the maintainer.
*** [do-build] Error code 1

Stop in /usr/ports/ports-mgmt/pkg.
*** [/usr/ports/ports-mgmt/pkg/work/.build_done.pkg._usr_local] Error code 1

Stop in /usr/ports/ports-mgmt/pkg.

Fix: 

in /usr/ports/ports-mgmt/pkg/work/pkg-1.2.2/libpkg/Makefile

Delete -Werror that is cause of problem 

-"CFLAGS+=        -Wall -Werror"
+"CFLAGS+=        -Wall"

Next:

set "WITHOUT_PKGNG=yes" into /etc/make.conf

/usr/ports/ports-mgmt/pkg # make install clean

set "WITH_PKGNG=yes" into /etc/make.conf

Confirmed:

# pkg info | grep pkg
pkg-1.2.2                      New generation package manager
pkgconf-0.9.3                  Utility to help to configure compiler and linker flags

I do not know, how "-Werror" is generating, in order to fix ready without error
How-To-Repeat: root@server:/usr/ports/ports-mgmt/pkg # make install clean