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

CMakeLists.txt

@@ -71,6 +71,10 @@ if(Backtrace_FOUND)
 	set(SWAY_Backtrace_HEADER "${Backtrace_HEADER}")
 endif()
 
+if (CMAKE_SYSTEM_NAME STREQUAL FreeBSD)
+	find_package(EpollShim REQUIRED)
+endif (CMAKE_SYSTEM_NAME STREQUAL FreeBSD)
+
 include(FeatureSummary)
 include(Manpage)
 include(GNUInstallDirs)