Fix build on FreeBSD

- Make sure CMake always finds absolute paths for Cairo, Pango and GdkPixbuf
- Add forgotten json-c include path to swaymsg/CMakeLists.txt
- Disable -Werror because of assert warnings
- Add correct /proc/pid/file path for FreeBSD
- Use libepoll-shim on FreeBSD
- Only use Linux capabilities on, well, Linux

swaymsg/CMakeLists.txt

@@ -2,6 +2,10 @@ add_executable(swaymsg
 	main.c
 )
 
+include_directories(
+	${JSONC_INCLUDE_DIRS}
+)
+
 target_link_libraries(swaymsg
 	sway-common
 	${JSONC_LIBRARIES}