Tests: Ensure project() clears unspecified properties

Add a unit test to ensure that various project-related variables are set
to empty when values are not provided in the project() call.

Tests/RunCMake/project/Omissions.cmake

@@ -0,0 +1,24 @@
+function(_expect_empty NAME)
+  if(NOT DEFINED ${NAME})
+    message(SEND_ERROR "Variable '${NAME}' is not defined")
+  elseif(NOT "${${NAME}}" STREQUAL "")
+    message(SEND_ERROR
+      "Variable '${NAME}' has value '${${NAME}}' (expected: empty)")
+  endif()
+endfunction()
+
+function(expect_empty SUFFIX)
+  _expect_empty(PROJECT_${SUFFIX})
+  _expect_empty(CMAKE_PROJECT_${SUFFIX})
+  _expect_empty(${PROJECT_NAME}_${SUFFIX})
+endfunction()
+
+project(Omissions LANGUAGES)
+expect_empty(VERSION)
+expect_empty(VERSION_MAJOR)
+expect_empty(VERSION_MINOR)
+expect_empty(VERSION_PATCH)
+expect_empty(VERSION_TWEAK)
+expect_empty(COMPAT_VERSION)
+expect_empty(DESCRIPTION)
+expect_empty(HOMEPAGE_URL)

Tests/RunCMake/project/RunCMakeTest.cmake

@@ -45,6 +45,7 @@ run_cmake(ProjectIsTopLevel)
 run_cmake(ProjectIsTopLevelMultiple)
 run_cmake(ProjectIsTopLevelSubdirectory)
 run_cmake(ProjectTwice)
+run_cmake(Omissions)
 run_cmake(VersionAndLanguagesEmpty)
 run_cmake(VersionEmpty)
 run_cmake(VersionInvalid)