Rework CMake support

pybind11_abseil/cmake/dependencies/CMakeLists.txt

@@ -0,0 +1,50 @@
+include(FetchContent)
+set(BUILD_SHARED_LIBS ON)
+set(BUILD_TESTING OFF)
+
+message(CHECK_START "Checking for external dependencies")
+list(APPEND CMAKE_MESSAGE_INDENT "  ")
+
+if(NOT TARGET absl::base)
+  if(USE_SYSTEM_ABSEIL)
+    # Version omitted, as absl only allows EXACT version matches
+    find_package(absl REQUIRED)
+  else()
+    message(CHECK_START "Fetching Abseil-cpp")
+    list(APPEND CMAKE_MESSAGE_INDENT "  ")
+    # ensure that abseil also installs itself, since we are using it in our public
+    # API
+    set(ABSL_ENABLE_INSTALL ON)
+    set(ABSL_PROPAGATE_CXX_STD ON)
+    set(ABSL_USE_SYSTEM_INCLUDES ON)
+    set(ABSL_BUILD_TESTING OFF)
+    FetchContent_Declare(
+      absl
+      GIT_REPOSITORY "https://github.com/abseil/abseil-cpp.git"
+      GIT_TAG "20240722.0"
+      GIT_SHALLOW TRUE)
+    FetchContent_MakeAvailable(absl)
+    list(POP_BACK CMAKE_MESSAGE_INDENT)
+    message(CHECK_PASS "fetched")
+  endif()
+endif()
+
+if(NOT TARGET pybind11::pybind11_headers)
+  if(USE_SYSTEM_PYBIND)
+    find_package(pybind11 2.13.6 REQUIRED)
+  else()
+    message(CHECK_START "Fetching pybind11")
+    list(APPEND CMAKE_MESSAGE_INDENT "  ")
+    FetchContent_Declare(
+      pybind11
+      GIT_REPOSITORY "https://github.com/pybind/pybind11.git"
+      GIT_TAG "v2.13.6"
+      GIT_SHALLOW TRUE)
+    FetchContent_MakeAvailable(pybind11)
+    list(POP_BACK CMAKE_MESSAGE_INDENT)
+    message(CHECK_PASS "fetched")
+  endif()
+endif()
+
+list(POP_BACK CMAKE_MESSAGE_INDENT)
+message(CHECK_PASS "all dependencies found")