The port persists to fail under 11.3, 11.4, 12-STABLE and CURRENT
.

[...]

=>> Building converters/wkhtmltopdf
build started at Wed Jun 24 15:50:05 UTC 2020
port directory: /usr/ports/converters/wkhtmltopdf
package name: wkhtmltopdf-0.12.5_5
building for: FreeBSD my.host.gdr 11.4-RELEASE FreeBSD 11.4-RELEASE 1104001 amd64
maintained by: pi@FreeBSD.org
Makefile ident:      $FreeBSD: head/converters/wkhtmltopdf/Makefile 516745 2019-11-04 21:36:13Z zeising $
Poudriere version: 3.3.4
Host OSVERSION: 1300099
Jail OSVERSION: 1104001
Job Id: 03

---Begin Environment---
SHELL=/bin/csh
OSVERSION=1104001
UNAME_v=FreeBSD 11.4-RELEASE 1104001
UNAME_r=11.4-RELEASE
BLOCKSIZE=K
MAIL=/var/mail/root
STATUS=1
HOME=/root
PATH=/sbin:/bin:/usr/sbin:/usr/bin:/usr/local/sbin:/usr/local/bin:/root/bin
LOCALBASE=/usr/local
USER=root
LIBEXECPREFIX=/usr/local/libexec/poudriere
POUDRIERE_VERSION=3.3.4
MAKEOBJDIRPREFIX=/pool/sources/11.4-RELENG/obj
MASTERMNT=/pool/poudriere/data/.m/11amd64-head-default/ref
POUDRIERE_BUILD_TYPE=bulk
PACKAGE_BUILDING=yes
SAVED_TERM=screen
PWD=/pool/poudriere/data/.m/11amd64-head-default/ref/.p/pool
P_PORTS_FEATURES=FLAVORS SELECTED_OPTIONS
MASTERNAME=11amd64-head-default
SCRIPTPREFIX=/usr/local/share/poudriere
OLDPWD=/pool/poudriere/data/.m/11amd64-head-default/ref/.p
SCRIPTPATH=/usr/local/share/poudriere/bulk.sh
POUDRIEREPATH=/usr/local/bin/poudriere
---End Environment---

---Begin Poudriere Port Flags/Env---
PORT_FLAGS=
PKGENV=
FLAVOR=
DEPENDS_ARGS=
MAKE_ARGS=
---End Poudriere Port Flags/Env---

---Begin OPTIONS List---
---End OPTIONS List---


[...]
2 warnings generated.
6 warnings generated.
10 warnings generated.
8 warnings generated.
rm -f libwkhtmltox.so.0.12.5 libwkhtmltox.so libwkhtmltox.so.0 libwkhtmltox.so.0.12
c++ -Wl,-rpath=/usr/local/lib/gcc9 -Wl,-O1 -pthread -shared -Wl,-soname,libwkhtmltox.so.0 -o libwkhtmltox.so.0.12.5 loadsettings.o logging.o multipageloader.o tempfile.o converter.o websettings.o reflect.o utilities.o pdfsettings.o pdfconverter.o outline.o tocstylesheet.o imagesettings.o imageconverter.o pdf_c_bindings.o image_c_bindings.o moc_multipageloader_p.o moc_converter_p.o moc_pdfconverter_p.o moc_imageconverter_p.o moc_pdf_c_bindings_p.o moc_image_c_bindings_p.o moc_converter.o moc_multipageloader.o moc_utilities.o moc_pdfconverter.o moc_imageconverter.o qrc_wkhtmltopdf.o   -L/usr/local/lib -L/wrkdirs/usr/ports/converters/wkhtmltopdf/work/wkhtmltopdf-0.12.5/build/qt/lib -L/wrkdirs/usr/ports/converters/wkhtmltopdf/work/wkhtmltopdf-0.12.5/build/qt/plugins/codecs -lqcncodecs -L/wrkdirs/usr/ports/converters/wkhtmltopdf/work/wkhtmltopdf-0.12.5/build/qt/lib -L/usr/local/lib -lqjpcodecs -lqkrcodecs -lqtwcodecs -lQtWebKit -lQtSvg -lQtXmlPatterns -lQtGui -ljpeg -lpng -lXrender -lfontconfig -lfreetype -lXext -lX11 -lQtNetwork -lssl -lcrypto -lQtCore -lz -lm  
/usr/bin/ld: cannot find -lQtWebKit
c++: error: linker command failed with exit code 1 (use -v to see invocation)
gmake[2]: *** [Makefile:164: ../../bin/libwkhtmltox.so.0.12.5] Error 1
gmake[2]: Leaving directory '/wrkdirs/usr/ports/converters/wkhtmltopdf/work/wkhtmltopdf-0.12.5/build/app/src/lib'
gmake[1]: *** [Makefile:49: sub-src-lib-all-ordered] Error 2
gmake[1]: Leaving directory '/wrkdirs/usr/ports/converters/wkhtmltopdf/work/wkhtmltopdf-0.12.5/build/app'
*** Error code 2

Stop.
make: stopped in /usr/ports/converters/wkhtmltopdf