meson: Don't use '' in link_with, ever

String arguments as elements in the array passed to the link_with
argument in meson's executable() functions are not valid and will end up
causing the build file generation to file. This actually ended up
exposing a bug in meson that caused it not to report where in the
meson.build file it was failing:

https://github.com/mesonbuild/meson/pull/2527

The proper way to have a variable that can contain either an empty link
target or an actual link target is:

some_target = []
if some_cond
    some_target = static_library(...)
endif

This way if some_cond is False, some_target gets set to [], gets passed
to executable() in the link_with array, and then gets removed by array
flattening.

This also unbreaks Xwayland builds with -Dglx=false, the thing that
originally made me notice this.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Jon Turney <jon.turney@dronecode.org.uk>

glx/meson.build

@@ -30,7 +30,7 @@ srcs_glx = [
     'xfont.c',
 ]
 
-libxserver_glx = ''
+libxserver_glx = []
 if build_glx
     libxserver_glx = static_library('libxserver_glx',
         srcs_glx,

hw/dmx/meson.build

@@ -43,7 +43,7 @@ dmx_dep = [
     dl_dep,
 ]
 
-dmx_glx = ''
+dmx_glx = []
 if build_glx
     srcs += 'dmx_glxvisuals.c'
     subdir('glxProxy')

meson.build

@@ -214,7 +214,7 @@ if int10 == 'auto'
     endif
 endif
 
-hal_dep = ''
+hal_dep = []
 if hal_option == 'auto'
     if not build_udev
         hal_dep = dependency('hal', required: false)
@@ -264,7 +264,7 @@ endif
 # XXX: Add more sha1 options, because Linux is about choice
 sha1_dep = nettle_dep
 
-xdmcp_dep = ''
+xdmcp_dep = []
 if get_option('xdmcp')
     xdmcp_dep = dependency('xdmcp')
 endif