Build: Update to C++17 version

The main rationale for this is for when fetching external GTest.
Currently we are stuck to GTest-1.12.1 which is the latest version
supporting C++11. Later versions require C++14.
The issue with GTest-1.12.1 is that its CMakeLists.txt has a
``cmake_minimum_required(VERSION 3.5)`` which causes the following
warning with modern CMake versions:
```
CMake Deprecation Warning at build/_deps/googletest-src/CMakeLists.txt:4 (cmake_minimum_required):
  Compatibility with CMake < 3.10 will be removed from a future version of
  CMake.
```

CMakeLists.txt

@@ -21,8 +21,8 @@ cmake_policy(SET CMP0054 NEW)
 
 # Set C++ version
 # Make CMAKE_CXX_STANDARD available as cache option overridable by user
-set(CMAKE_CXX_STANDARD 11
-  CACHE STRING "C++ standard version to use (default is 11)")
+set(CMAKE_CXX_STANDARD 17
+  CACHE STRING "C++ standard version to use (default is 17)")
 message(STATUS "Requiring C++${CMAKE_CXX_STANDARD}")
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_CXX_EXTENSIONS OFF)

docs/source/install.rst

@@ -146,7 +146,7 @@ Build requirements
 ++++++++++++++++++
 
 - C99 compiler
-- C++11 compiler
+- C++17 compiler
 - CMake >= 3.16
 - SQLite3 >= 3.11: headers and library for target architecture, and sqlite3 executable for build architecture
 - libtiff >= 4.0 (optional but recommended)
@@ -160,7 +160,7 @@ Test requirements
 
 These are only required if testing is built (see :option:`BUILD_TESTING`, default ON)
 
-- GoogleTest (GTest) >= 1.8.1; if not found and :option:`TESTING_USE_NETWORK` is ON, then version 1.12.1 is fetched from GitHub and locally installed
+- GoogleTest (GTest) >= 1.8.1; if not found and :option:`TESTING_USE_NETWORK` is ON, then version 1.15.2 is fetched from GitHub and locally installed
 - Python >= 3.7
 - `importlib_metadata <https://pypi.org/project/importlib-metadata/>`_ only needed for Python 3.7
 - One of either `PyYAML <https://pypi.org/project/PyYAML/>`_ or `ruamel.yaml <https://pypi.org/project/ruamel.yaml/>`_

test/unit/CMakeLists.txt

@@ -60,10 +60,12 @@ elseif(HAS_NETWORK)
     cmake_policy(SET CMP0135 NEW)  # for DOWNLOAD_EXTRACT_TIMESTAMP option
   endif()
 
+  set(GTEST_VERSION "1.15.2")
+
   include(FetchContent)
   FetchContent_Declare(
     googletest
-    URL https://github.com/google/googletest/archive/refs/tags/release-1.12.1.zip
+    URL https://github.com/google/googletest/archive/refs/tags/release-${GTEST_VERSION}.zip
     EXCLUDE_FROM_ALL  # ignored before CMake 3.28
   )