cet_cmake_env() fails to set correct output directories when included implicitly via subproject #39

Description

When cetmodules is included as a subproject (e.g., via FetchContent), the cet_cmake_env() macro may fail to set the correct CMAKE_LIBRARY_OUTPUT_DIRECTORY in the parent project.

Root Cause

In Modules/CetCMakeEnv.cmake, include(GNUInstallDirs) is called at the top level of the file. When cetmodules is added via add_subdirectory() (which FetchContent_MakeAvailable does), this include is executed in the cetmodules scope.

Because GNUInstallDirs defines variables like CMAKE_INSTALL_LIBDIR in the current scope, these variables are not available to the parent project. When the parent project later calls cet_cmake_env(), the project_variable(LIBRARY_DIR ...) call uses an undefined CMAKE_INSTALL_LIBDIR, resulting in phlex_LIBRARY_DIR (or equivalent) being an empty string. Consequently, CMAKE_LIBRARY_OUTPUT_DIRECTORY is set to ${PROJECT_BINARY_DIR} instead of the expected ${PROJECT_BINARY_DIR}/lib.

This issue is masked if the parent project explicitly calls include(CetCMakeEnv), as that forces GNUInstallDirs to be processed in the parent's scope. However, relying on this is fragile and counter-intuitive since cet_cmake_env() is designed to bootstrap the environment.

Proposed Solution

Move the include(GNUInstallDirs) call from the top level of Modules/CetCMakeEnv.cmake into the cet_cmake_env() macro. This ensures that the necessary installation directory variables are defined in the scope of the project that is actually initializing its environment.

Suggested Change:

--- a/Modules/CetCMakeEnv.cmake
+++ b/Modules/CetCMakeEnv.cmake
@@ -55,8 +55,6 @@
   endif()
 
- # See https://cmake.org/cmake/help/latest/module/GNUInstallDirs.html.
- include(GNUInstallDirs)
  ##################
 
  define_property(TARGET PROPERTY CET_EXEC_LOCATION
@@ -109: macro(cet_cmake_env)
+  # See https://cmake.org/cmake/help/latest/module/GNUInstallDirs.html.
+  include(GNUInstallDirs)
+
   # project() must have been called first.