Simplify compile tests.

CMakeLists.txt

@@ -28,12 +28,6 @@ add_subdirectory(doc)
 
 enable_testing()
 
-# Remove the CMake cache in the test directory so that CMake
-# performed full configuration. Otherwise compile-test will
-# just used cached information.
-add_test(prepare-test ${CMAKE_COMMAND}
-  -E remove ${CMAKE_CURRENT_BINARY_DIR}/test/CMakeCache.txt)
-
 add_test(compile-test ${CMAKE_CTEST_COMMAND}
   --build-and-test
   "${CMAKE_CURRENT_SOURCE_DIR}/test"

test/CMakeLists.txt

@@ -10,10 +10,13 @@ function (expect_compile_error code)
     ${code}
   }
   " compiles)
-  if (compiles)
+  set (does_compile ${compiles})
+  # Unset the CMake cache variable compiles. Otherwise the compile test will
+  # just used cached information next time it runs.
+  unset(compiles CACHE)
+  if (does_compile)
     error("No compile error for: ${code}")
   endif ()
-  unset(compiles CACHE)
 endfunction ()
 
 # Writing a wide character to a character stream Writer is forbidden.