https://bugs.gentoo.org/978866#c7

Thanks to Bernd Feige.

--- a/rake.d/gtest.rb
+++ b/rake.d/gtest.rb
@@ -5,12 +5,12 @@ $gtest_internal = c(:GTEST_TYPE) == "int
 
 namespace :tests do
   desc "Build the unit tests"
-  task :unit => $gtest_apps.collect { |app| "tests/unit/#{app}/#{app}" + c(:EXEEXT) }
+  task :unit => $gtest_apps.select { |app| app != 'gui' || c?(:BUILD_GUI) }.collect { |app| "tests/unit/#{app}/#{app}" + c(:EXEEXT) }
 
   desc "Build and run the unit tests"
   task :run_unit => 'tests:unit' do
     runtime, args = $building_for[:windows] ? [ "wine", "--gtest_color=no | tr -d '\n'" ] : [ "", "" ]
-    $gtest_apps.each { |app| run "LC_ALL=C #{runtime} ./tests/unit/#{app}/#{app}#{c(:EXEEXT)} #{args}" }
+    $gtest_apps.select { |app| app != 'gui' || c?(:BUILD_GUI) }.each { |app| run "LC_ALL=C #{runtime} ./tests/unit/#{app}/#{app}#{c(:EXEEXT)} #{args}" }
   end
 
   if !c(:VALGRIND).to_s.empty?