This patch applies from WRKSRC_SUBDIR, ie. from cpp/ , but even
when building in that subdir, the tools/ directory is built
as well. That is why the patch applies to ..

- use system abseil instead of fetching it on our own.
- force C++17 because abseil has it, too.

--- ../tools/cpp/CMakeLists.txt.orig	2022-01-11 10:41:37.000000000 +0100
+++ ../tools/cpp/CMakeLists.txt	2022-01-22 17:06:39.284013000 +0100
@@ -18,7 +18,7 @@
 
 # Pick the C++ standard to compile with.
 # Abseil currently supports C++11, C++14, and C++17.
-set(CMAKE_CXX_STANDARD 11)
+set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
 project (generate_geocoding_data)
@@ -28,27 +28,11 @@
 include (gtest.cmake)
 include (FetchContent)
 
-# Downloading the abseil sources.
-FetchContent_Declare(
-    abseil-cpp
-    GIT_REPOSITORY  https://github.com/abseil/abseil-cpp.git
-    GIT_TAG         origin/master
-)
+find_package(absl REQUIRED)
 
-# Building the abseil binaries
-FetchContent_GetProperties(abseil-cpp)
-if (NOT abseil-cpp_POPULATED)
-    FetchContent_Populate(abseil-cpp)
-endif ()
-
-if (NOT abseil-cpp_POPULATED)
-   message (FATAL_ERROR "Could not build abseil-cpp binaries.")
-endif ()
-
 # Safeguarding against any potential link errors as mentioned in
 # https://github.com/abseil/abseil-cpp/issues/225
 set(CMAKE_POSITION_INDEPENDENT_CODE TRUE)
-add_subdirectory(${abseil-cpp_SOURCE_DIR} ${abseil-cpp_BINARY_DIR})
 
 find_or_build_gtest ()
 set (