DSO missing from command line

[d.healey]

I'm getting this message when building a standalone app (doesn't happen with the plugin).

/usr/bin/ld: build/intermediate/Release/include_juce_gui_basics_e3f79785.o: undefined reference to symbol 'XConvertSelection'
//usr/lib/x86_64-linux-gnu/libX11.so.6: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status

On my laptop I also get the same message but referring to libXext.so.

I managed to get it to link by adding some extra linker flags.
-lX11
-lXext