If I pass -DKACTIVITIES_LIBRARY_ONLY:BOOL=TRUE to the build and boost isn't installed I get the error below. If I'm not mistaken https://quickgit.kde.org/?p=kactivities.git&a=commit&h=9274281f68b9fc387848a4b49b36934b6b216b3c introduced this.
I could probably come up with a patch moving the find_package(Boost) stuff outside the "if (NOT KACTIVITIES_LIBRARY_ONLY)" scope.

[ 48%] Building CXX object src/lib/core/CMakeFiles/KF5Activities.dir/__/__/utils/dbusfuture_p.cpp.o
cd /var/tmp/paludis/build/kde-frameworks-kactivities-scm/work/build/src/lib/core && /usr/bin/x86_64-pc-linux-gnu-c++   -DKF5Activitie
s_EXPORTS -DQT_CORE_LIB -DQT_DBUS_LIB -DQT_NO_DEBUG -DTRANSLATION_DOMAIN=\"kactivities5\" -D_GNU_SOURCE -D_LARGEFILE64_SOURCE -pipe -
O2 -DNDEBUG -DQT_NO_DEBUG -std=c++0x -fno-exceptions -Wall -Wextra -Wcast-align -Wchar-subscripts -Wformat-security -Wno-long-long -W
pointer-arith -Wundef -Wnon-virtual-dtor -Woverloaded-virtual -Werror=return-type -fPIC -fvisibility=hidden -fvisibility-inlines-hidd
en -I/var/tmp/paludis/build/kde-frameworks-kactivities-scm/work/build/src/lib/core -I/var/tmp/paludis/build/kde-frameworks-kactivitie
s-scm/work/kactivities-scm/src/lib/core -I/var/tmp/paludis/build/kde-frameworks-kactivities-scm/work/kactivities-scm/src -isystem /us
r/include/qt5 -isystem /usr/include/qt5/QtCore -isystem /usr/lib/qt5/mkspecs/linux-g++ -isystem /usr/include/qt5/QtDBus -isystem /usr
/include/KF5/KDBusAddons -isystem /usr/include/KF5    -fPIC -o CMakeFiles/KF5Activities.dir/__/__/utils/dbusfuture_p.cpp.o -c /var/tm
p/paludis/build/kde-frameworks-kactivities-scm/work/kactivities-scm/src/utils/dbusfuture_p.cpp
In file included from /var/tmp/paludis/build/kde-frameworks-kactivities-scm/work/kactivities-scm/src/lib/core/manager_p.cpp:34:0:
/var/tmp/paludis/build/kde-frameworks-kactivities-scm/work/kactivities-scm/src/utils/continue_with.h:14:30: fatal error: boost/optional.hpp: No such file or directory
compilation terminated.
src/lib/core/CMakeFiles/KF5Activities.dir/build.make:253: recipe for target 'src/lib/core/CMakeFiles/KF5Activities.dir/manager_p.cpp.o' failed
make[2]: *** [src/lib/core/CMakeFiles/KF5Activities.dir/manager_p.cpp.o] Error 1
make[2]: *** Waiting for unfinished jobs....
make[2]: Leaving directory '/var/tmp/paludis/build/kde-frameworks-kactivities-scm/work/build'
CMakeFiles/Makefile2:157: recipe for target 'src/lib/core/CMakeFiles/KF5Activities.dir/all' failed
make[1]: Leaving directory '/var/tmp/paludis/build/kde-frameworks-kactivities-scm/work/build'
make[1]: *** [src/lib/core/CMakeFiles/KF5Activities.dir/all] Error 2
Makefile:163: recipe for target 'all' failed
make: *** [all] Error 2

Reproducible: Always