link_with: isn't flattened

[jon-turney]

extracted from xserver meson.build, with the maze of conditionals removed for clarity:

project('test', 'c')

libxserver_dri3 = []
libxserver = [ libxserver_dri3 ]

executable('Xephyr', 'xephyr.c', link_with: [ libxserver ])

fails with

Meson encountered an error in file meson.build, line 6, column 0:
Link target [] is not library.