The problem appears to be that the build has requested the OS X 10.8 deployment target, but is using a function not available until OS X 10.10. A simple solution might be for the build not to request the OS X 10.8 deployment target.
:info:build /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang++ -c -pipe -stdlib=libc++ -ffunction-sections -Os -fPIC -std=c++1z -fapplication-extension -arch x86_64 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk -mmacosx-version-min=10.8 -fvisibility=hidden -fvisibility-inlines-hidden -fno-exceptions -Wall -W -Werror=unguarded-availability -Werror=unguarded-availability-new -Werror=unsupported-availability-guard -DQT_VERSION_STR='"5.10.0"' -DQT_VERSION_MAJOR=5 -DQT_VERSION_MINOR=10 -DQT_VERSION_PATCH=0 -DQT_BOOTSTRAPPED -DQT_NO_CAST_TO_ASCII -DQT_NO_FOREACH -DQT_NO_CAST_FROM_ASCII -DQT_NO_NARROWING_CONVERSIONS_IN_CONNECT -DQT_BUILD_BOOTSTRAP_LIB -DQT_BUILDING_QT -DQT_ASCII_CAST_WARNINGS -DQT_MOC_COMPAT -DQT_USE_QSTRINGBUILDER -DQT_DEPRECATED_WARNINGS -DQT_DISABLE_DEPRECATED_BEFORE=0x050000 -DQT_NO_EXCEPTIONS -D_LARGEFILE64_SOURCE -D_LARGEFILE_SOURCE -DQT_NO_DEBUG -I. -I/opt/digikam/var/macports/build/_opt_digikam_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_aqua_qt5/qt5-qtbase/work/qtbase-everywhere-src-5.10.0/lib/QtCore.framework/Headers -I/opt/digikam/var/macports/build/_opt_digikam_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_aqua_qt5/qt5-qtbase/work/qtbase-everywhere-src-5.10.0/lib/QtCore.framework/Headers/5.10.0 -I/opt/digikam/var/macports/build/_opt_digikam_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_aqua_qt5/qt5-qtbase/work/qtbase-everywhere-src-5.10.0/lib/QtCore.framework/Headers/5.10.0/QtCore -I/opt/digikam/var/macports/build/_opt_digikam_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_aqua_qt5/qt5-qtbase/work/qtbase-everywhere-src-5.10.0/lib/QtXml.framework/Headers -I/opt/digikam/var/macports/build/_opt_digikam_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_aqua_qt5/qt5-qtbase/work/qtbase-everywhere-src-5.10.0/lib/QtXml.framework/Headers/5.10.0 -I/opt/digikam/var/macports/build/_opt_digikam_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_aqua_qt5/qt5-qtbase/work/qtbase-everywhere-src-5.10.0/lib/QtXml.framework/Headers/5.10.0/QtXml -I../../../include -I../../../include/QtCore -I../../../include/QtCore/5.10.0 -I../../../include/QtCore/5.10.0/QtCore -I../../../include/QtXml -I../../../include/QtXml/5.10.0 -I../../../include/QtXml/5.10.0/QtXml -I../../../mkspecs/macx-clang -o .obj/qcore_mac_objc.o ../../corelib/kernel/qcore_mac_objc.mm
:info:build ../../corelib/io/qfilesystemengine_unix.cpp:168:41: error: 'LSCopyDefaultApplicationURLForURL' is only available on macOS 10.10 or newer [-Werror,-Wunguarded-availability]
:info:build QCFType<CFURLRef> application = LSCopyDefaultApplicationURLForURL(url,
:info:build ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
