Adjust CMAKE_PREFIX_PATH environment variable for COMPILE_ONLY tests.

Makes it possible to run COMPILE_ONLY tests without users needing to
adjust the CMAKE_PREFIX_PATH environment variable in their own
environments.

Modules/CetTest.cmake

@@ -687,6 +687,19 @@ test ${test} must be defined already to be specified as a fixture for ${CET_TARG
             ENVIRONMENT_MODIFICATION "${CET_TEST_ENV_MODIFICATION}")
         endif()
       endif()
+      if (CET_COMPILE_ONLY)
+        _transitive_binary_directories(${CETMODULES_CURRENT_PROJECT_NAME})
+        list(JOIN TRANSITIVE_BINARY_DIRS ":" DIRS_FOR_PREFIX_PATH)
+        set(TEST_CMAKE_PREFIX_PATH "CMAKE_PREFIX_PATH=path_list_prepend:${DIRS_FOR_PREFIX_PATH}")
+        get_test_property(${target} ENVIRONMENT_MODIFICATION CET_TEST_ENV_TMP)
+        if (CET_TEST_ENV_TMP)
+          set_tests_properties(${target} PROPERTIES
+            ENVIRONMENT_MODIFICATION "${TEST_CMAKE_PREFIX_PATH};${CET_TEST_ENV_TMP}")
+        else()
+          set_tests_properties(${target} PROPERTIES
+            ENVIRONMENT_MODIFICATION "${TEST_CMAKE_PREFIX_PATH}")
+        endif()
+      endif()
       if (CET_REF)
         get_test_property(${target} REQUIRED_FILES REQUIRED_FILES_TMP)
         if (REQUIRED_FILES_TMP)
@@ -1087,3 +1100,15 @@ function(_update_defined_test_groups)
     FORCE
     )
 endfunction()
+
+function(_transitive_binary_directories PKG)
+  set(TRANSITIVE_BINARY_DIRS_TMP ${${PKG}_BINARY_DIR})
+  foreach(DEP IN LISTS CETMODULES_FIND_DEPS_PNAMES_PROJECT_${PKG})
+    if (${DEP}_IN_TREE)
+      _transitive_binary_directories(${DEP})
+      list(APPEND TRANSITIVE_BINARY_DIRS_TMP ${TRANSITIVE_BINARY_DIRS})
+    endif()
+  endforeach()
+  list(REMOVE_DUPLICATES TRANSITIVE_BINARY_DIRS_TMP)
+  set(TRANSITIVE_BINARY_DIRS ${TRANSITIVE_BINARY_DIRS_TMP} PARENT_SCOPE)
+endfunction()