RFC 7: Drop Autotools, maintain CMake (#2880)

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+.. _rfc7:
+
+====================================================================
+PROJ RFC 7: Drop Autotools, maintain CMake
+====================================================================
+
+:Author: Mike Taves
+:Contact: mwtoews@gmail.com
+:Status: Adopted
+:Implementation target: PROJ 9.0
+:Last Updated: 2021-10-27
+
+Summary
+-------------------------------------------------------------------------------
+
+This RFC proposes to drop Autotools for PROJ 9.0, and to maintain CMake
+for build automation, testing and packaging. This will reduce the overall
+maintenance for PROJ and will enable the library to be integrated into other
+projects that use CMake.
+
+Background
+-------------------------------------------------------------------------------
+
+Here is a short timeline of the build tools used for PROJ:
+
+- Throughout the mid-1990s, Gerald Evenden maintained a Unix build system with
+  a few scripts (some derived from Autoconf), and Makefile templates.
+- In 2000, Frank Warmerdam wrote Autoconf and Automake configurations for
+  PROJ 4.4.0.
+- This was followed by a NMake configuration to build PROJ 4.4.2 for Windows.
+- In 2014, a CMake build setup was introduced by Howard Butler for
+  PROJ 4.9.0RC1. The CMake configuration was improved for the 4.9.1 release,
+  but not considered at feature parity with the Autotools builds at the time.
+- The NMake build setup was removed for PROJ 6.0.0, as its functionality had
+  been replaced by CMake.
+
+Motivation
+-------------------------------------------------------------------------------
+
+The primary motivation in removing Autotools is to reduce the burden of
+maintaining multiple build configurations, which requires developers to be
+familiar with different tools and configuration files. There are several other
+benefits in maintaining a single build system:
+
+- Remove extra configuration and m4 macro files from source repository,
+- Simplify scripts used for running tests for CI services (GitHub Actions,
+  TravisCI),
+- Reduce compilation time (and carbon footprint) used for testing on CI
+  services,
+- Ease development effort, particularly with new contributors.
+
+Why drop Autotools?
+-------------------------------------------------------------------------------
+
+The GNU Build System or Autotools consist of a suite of tools including
+Autoconf and Automake, which can be used to build software on Unix-like
+systems. These tools are not cross-platform, and do not naively integrate
+with development environments on Microsoft Windows. Furthermore, the existing
+PROJ Autotools builds do not install the CMake configuration files required to
+find PROJ from other projects that use CMake
+(`#2546 <https://github.com/OSGeo/PROJ/issues/2546>`_).
+
+Why use CMake?
+-------------------------------------------------------------------------------
+
+CMake is an open source cross-platform tool for build automation, testing and
+packaging of software. It does not directly compile the software, but manages
+the build process using generators, including Unix Makefiles and Ninja among
+other command-based and IDE tools. The CMake software has been under active
+development since its origins in 2000. The CMake language is carefully
+developed with backwards-compatible policies that aim to provide consistent
+behaviour across different versions. CMake is currently the preferred build
+tool for PROJ for the following reasons:
+
+- It has existed in the PROJ code base since 2014, and is familiar to active
+  PROJ contributors,
+- It can install configuration files that can be used by other software that
+  use CMake to find PROJ for linking via ``find_package()``,
+- CMake configurations are used in 3rd-party binary packages of PROJ,
+  including conda-forge and vcpkg,
+- It can be used to build PROJ on all major operating systems and compiler
+  combinations (where compatible),
+- It has integration with modern IDEs and tools, including
+  Microsoft Visual Studio and Qt Creator.
+
+Why not CMake?
+-------------------------------------------------------------------------------
+
+Other modern cross-platform build systems exist, including Meson and Bazel,
+which have many advantages over CMake. However, they are currently not widely
+used by active PROJ contributors. This RFC should not restrict future build
+system configurations being introduced to PROJ, if they are proven to have
+advantages to CMake over time.
+
+Potential impacts
+-------------------------------------------------------------------------------
+
+Binary packagers that currently rely on Autotools would obviously need to
+transition building and testing PROJ with CMake. Issues related to
+multiarch builds of PROJ may become apparent, which can be patched and/or
+reported to PROJ developers. One feature of Autotools is that both static and
+dynamic (shared) libraries are built, which packagers may distribute. This
+feature is currently not set-up for PROJ, as it would need to be configured
+and built twice.
+
+End-users that use binary packages of PROJ should not be impacted. PROJ should
+be discoverable via both pkg-config and CMake's ``find_package()``.
+Other projects that use Autotools will continue to work as expected,
+linking statically or dynamically to PROJ built by CMake.
+
+Transition plan
+-------------------------------------------------------------------------------
+
+If this proposal is approved, the following tasks should be completed:
+
+- Rewrite CI tests to only use CMake for packaging, building, testing,
+  installation and post-install tests,
+- Remove files only used by Autotools, also update ``.gitignore``,
+- Update documentation and ``HOWTORELEASE`` notes.
+
+Related issues will be tracked on GitHub with a tag
+`RFC7: Autotools→CMake <https://github.com/OSGeo/PROJ/labels/RFC7%3A%20Autotools%E2%86%92CMake>`_.
+
+Adoption status
+-------------------------------------------------------------------------------
+
+The RFC was adopted on 2021-10-26 with +1's from the following PSC members
+
+* Kristian Evers
+* Even Rouault
+* Howard Butler
+* Thomas Knudsen
+* Kurt Schwehr
+* Charles Karney
+* Thomas Knudsen