FTBFS with Vala 0.42

[jtojnar]

Building this fails on Nix with Vala 0.42:

unpacking sources
unpacking source archive /nix/store/k3p7h9klz5p0kzx534icndkvgz5h702r-source
source root is source
patching sources
configuring
Setting top to                           : /build/source 
Setting out to                           : /build/source/build 
Checking for 'gcc' (c compiler)          : gcc 
Checking for program valac-0.12          : not found 
Checking for program valac               : /nix/store/99qldd1s09hg1924qma60gzipsda0kvv-vala-0.42.0/bin/valac 
Checking for valac version >= (0, 12, 0) : (0, 42, 0) 
Checking for program pkg-config          : /nix/store/dj3sdmwgfqbnn09bdf6spzw5p58ck0jq-pkg-config-0.29.2/bin/pkg-config 
Checking for 'glib-2.0' >= 2.10          : yes 
Checking for 'gtk+-2.0' >= 2.16          : yes 
Checking for 'libxfce4panel-1.0' >= 4.6  : yes 
Checking for 'libwnck-1.0' >= 2.20       : yes 
'configure' finished successfully (0.168s)
building
Waf: Entering directory `/build/source/build'
[1/6] valac: src/namebar.vala src/preferences.vala src/theme.vala -> build/src/namebar.c build/src/preferences.c build/src/theme.c
[2/6] subst: data/namebar.desktop.in -> build/data/namebar.desktop
gtk+-2.0.vapi:5873.40-5873.42: warning: Gtk is deprecated. Use gtk+-3.0
/build/source/src/namebar.vala:53.5-53.22: warning: NamebarPlugin.size hides inherited property `Xfce.PanelPlugin.size'. Use the `new' keyword if hiding was intentional
/build/source/src/namebar.vala:54.5-54.24: warning: NamebarPlugin.expand hides inherited property `Xfce.PanelPlugin.expand'. Use the `new' keyword if hiding was intentional
/build/source/src/namebar.vala:369.47-369.50: warning: Wnck is deprecated. Use libwnck-3.0
/build/source/src/namebar.vala:370.46-370.49: warning: Wnck is deprecated. Use libwnck-3.0
/build/source/src/namebar.vala:388.46-388.49: warning: Wnck is deprecated. Use libwnck-3.0
/build/source/src/namebar.vala:389.46-389.49: warning: Wnck is deprecated. Use libwnck-3.0
/build/source/src/namebar.vala:490.47-490.50: warning: Wnck is deprecated. Use libwnck-3.0
/build/source/src/namebar.vala:491.46-491.49: warning: Wnck is deprecated. Use libwnck-3.0
/build/source/src/preferences.vala:290.23-290.50: error: The name `FILE_ATTRIBUTE_STANDARD_NAME' does not exist in the context of `PrefDialog.find_themes'
/build/source/src/preferences.vala:176.27-176.29: warning: Gdk is deprecated. Use gdk-3.0
Waf: Leaving directory `/build/source/build'
Build failed
 -> task failed (exit status 1): 
        {task 140737270219088: valac namebar.vala,preferences.vala,theme.vala -> namebar.c,preferences.c,theme.c}
''
builder for '/nix/store/617xy9q45y08q5i4w64ww8nbifcxvm0s-xfce4-namebar-plugin-0.3.1.drv' failed with exit code 1