Temporary, see:

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=231108

--- GNUmakefile.orig	2018-09-02 22:04:19 UTC
+++ GNUmakefile
@@ -5,7 +5,7 @@ ifndef OBS_LIB
 OBS_LIB = /usr/lib
 endif
 
-CXXFLAGS = -std=c++11 -Wall -g -fPIC -I$(OBS_INCLUDE) -I./src $(shell pkg-config --cflags Qt5Widgets) $(shell pkg-config --cflags Qt5WebKit) $(shell pkg-config --cflags Qt5WebKitWidgets)
+CXXFLAGS += -std=c++11 -Wall -g -fPIC -I$(OBS_INCLUDE) -I./src $(shell pkg-config --cflags Qt5Widgets) $(shell pkg-config --cflags Qt5WebKit) -I%%LOCALBASE%%/include/qt5/QtWebKitWidgets
 CXX      ?= c++
 RM       = /bin/rm -rf
 LDFLAGS  = -L$(OBS_LIB)