bumping the minimum CMake version (to 3.18 in April 2026) #18704

At the moment we require CMake 3.7, released 2018-Dec-03 (7y old version).
Supporting this and other older versions needs developing and maintaining
a growing number of hacks and fallback code.

The reason for 3.7 is being the version used in Debian Stretch:
https://sources.debian.org/src/cmake/, which is an extented LTS release
(Ref 376cd67).

Old version lack these useful features:

• LTO support: 3.9+
• C++11 compiler required to build cmake itself: 3.10+
• performance improvements: 3.11+ [link]
• better pkg-config support: 3.12+
• OBJECT target: 3.12+
• FindPython: 3.12+
• option de-duplication via SHELL:: 3.12+
• -S/-B support at invocation: 3.13+
• lib directory support in interface targets: 3.13+
• target_link_options(), LINK_OPTIONS, and other script niceties: 3.13+
• FetchContent: 3.14+ (in integration tests)
• --verbose option: 3.14+ (older versions needed generator-specific solutions, e.g. for GNU Makefile VERBOSE=1 env, and less obvious ones for the others, if any.)
• --install support: 3.15+ (in integration tests)
• list(PREPEND ...): 3.15+
• CMAKE_GENERATOR env: 3.15+
• FindGnuTLS with GnuTLS::GnuTLS imported target: 3.16+
• unity: 3.16+
• Ninja: 3.17+
• fixed libdir propagation: 3.17+
• fixed imported global issues: 3.17+
• FindPython improvements: 3.14–3.18+
• supports OpenSSL 3: 3.18+
• supports file(CONFIGURE): 3.18+
• fixes to support < characters in input in file(CONFIGURE(): 3.19+
• allows reading the LOCATION target property without the error: INTERFACE_LIBRARY targets may only have whitelisted properties. The property "LOCATION" is not allowed.: 3.19+
• better support to debug via dumping target props: 3.19+
• CMake no longer restricts custom property names, e.g. INTERFACE_LIBCURL_PC_MODULES in curl. It can be renamed to LIBCURL_PC_MODULES (as was planned before bumping into this issue): 3.19+ https://gitlab.kitware.com/cmake/cmake/-/commit/e3a1a688eac50bda0fd2844b83205aa5fd1de5ad
• presets: 3.19+
• pre-built binaries for macOS ARM64: 3.19.2+
• pre-built binaries for Linux ARM64: 3.19.3+
• cmake_path(): 3.20+
• COMPILE_WARNING_AS_ERROR: 3.24+
• ZLIB_USE_STATIC_LIBS: 3.24+
• blocks for scoping: 3.25+
• cmake 3.25 loses support for the VS2010 generator: https://cmake.org/cmake/help/latest/generator/Visual%20Studio%2010%202010.html
• newer versions seem to build faster
• CMakeConfigureLog.yaml: 3.26+
• Visual Studio 18 2026 generator: 4.2+
• built-in way to dump variables, target properties, and more: 4.5+

Some of these feature would make the long-time pending #16973 PR
more practical and, possibly, more robust.

The speed boosters are unity, ninja, object targets. Also internal perf
changes in e.g. 3.11+.

HP-UX doesn't support 3.10+, according to: #18611

Some dependencies already require newer versions: nghttp2: 3.14,
boringssl, brotli: 3.15, modern Xcode: 3.15, libressl: 3.16, ngtcp2, nghttp3: 3.20

Requiring 3.17 would be ideal, released 2019-Nov-26. Going with
3.13 would already be helpful, it was released just one year prior:
2018-Nov-21. (3.14, 3.15, 3.16 were all released in 2019 apparently.)

Bump history:
2026-04 3.7 (10y old) → 3.18 (6y old): proposed
2022-12-26 3.2 (7y old) → 3.7 (6y old): dfbe035 #10161
2020-05-10 3.0 (6y old) → 3.2 (5y old): ad64169 #5358
2020-02-24 policies NEW bump to 3.16: fc9312f #4975
2018-09-21 3.4 (3y old) → 3.0 (4y old): 518ed51 #3055
2018-07-19 2.8.12 (5y old) → 3.4 (3y old): f826b4c #2753
2017-09-10 2.8 (8y old) → 2.8.12 (4y old): 1cb4f5d #1879
2014-07-30 2.6.2 (6y old) → 2.8 (5y old): 14aa8f0
2009-04-02 2.6.2 (initial) (3y old): 4c5307b

CMake release dates:
2020-10-06 v3.18.4
2020-07-15 v3.18.0
2020-03-20 v3.17.0
2019-12-19 v3.16.2
2018-11-20 v3.13.0
2016-11-11 v3.7.0
2015-11-12 v3.4.0
2015-03-04 v3.2.0
2014-06-10 v3.0.0
2013-10-08 v2.8.12
2009-11-13 v2.8.0
2008-09-24 v2.6.2

Refs:
https://cliutils.gitlab.io/modern-cmake/chapters/intro/dodonot.html