Guard functional_cpp population for CMake older than 3.18

FetchContent's SOURCE_SUBDIR handling was added in CMake 3.18, but the
project's minimum is 3.14; on 3.14-3.17, MakeAvailable would have
add_subdirectory'd functional_cpp's top-level CMakeLists.txt, pulling its
own src/tests into this build. Branch on the CMake version: 3.18+ keeps the
SOURCE_SUBDIR MakeAvailable path, older versions populate the sources via
FetchContent_Populate, which never calls add_subdirectory at all. The
deprecated-in-3.30 Populate call sits behind the VERSION_LESS 3.18 check,
so modern CMake never executes it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NUt3c1wdseRtRSSXfg3MCK

Author: claude

CMakeLists.txt

@@ -26,16 +26,27 @@ FetchContent_MakeAvailable(googletest)
 
 # functional_cpp provides fcpp::optional_t, the nullable-field representation
 # (std::optional under C++17 and later, an equivalent C++11 fallback otherwise).
-# It is consumed header-only: SOURCE_SUBDIR points at the header directory, which
-# contains no CMakeLists.txt, so FetchContent only downloads the sources without
-# building functional_cpp's own library and test targets.
+# It is consumed header-only: only its sources are downloaded, and functional_cpp's
+# own library and test targets are never added to this build.
 FetchContent_Declare(
   functional_cpp
   GIT_REPOSITORY https://github.com/jkalias/functional_cpp.git
   GIT_TAG 1.1.1
   SOURCE_SUBDIR include
 )
-FetchContent_MakeAvailable(functional_cpp)
+if(CMAKE_VERSION VERSION_LESS 3.18)
+  # FetchContent's SOURCE_SUBDIR support was added in CMake 3.18; on older versions
+  # MakeAvailable would add_subdirectory functional_cpp's top-level CMakeLists.txt,
+  # pulling its src/tests into this build. Populate without adding any subdirectory.
+  FetchContent_GetProperties(functional_cpp)
+  if(NOT functional_cpp_POPULATED)
+    FetchContent_Populate(functional_cpp)
+  endif()
+else()
+  # SOURCE_SUBDIR points at the header directory, which contains no CMakeLists.txt,
+  # so MakeAvailable populates the sources without calling add_subdirectory
+  FetchContent_MakeAvailable(functional_cpp)
+endif()
 include_directories(${functional_cpp_SOURCE_DIR}/include)
 
 # Set compiler settings based on the current platform