build(deps): Raise minimum CMake dependency from 3.12 to 3.15

[lgritz]

This bump brings access to the following interesting features (not a comprehensive list):

• 3.13 Nov 21, 2018

  • -B, -S let you specify source and build directories without the "cd" dance
  • target_link_options()
  • ctest --progress / CTEST_PROGRESS_OUTPUT

• 3.14 Mar 14, 2019

  • cmake --build now supports -v/--verbose
  • file(CREATE_LINK )

• 3.15 Jul 17, 2019

  • env CMAKE_GENERATOR can be used instead of -G
  • when using --build, the --target can now list multiple targets (and also can be -t)
  • --install can be used separately without running through the whole build
  • message() now supports NOTICE, VERBOSE, DEBUG and TRACE.
  • CMAKE_FIND_PACKAGE_PREFER_CONFIG variable
  • Enable C++20 for Apple Clang

Make sure all CI cases have a new enough cmake

Use message(VERBOSE ...) feature to simplify lots of if blocks.

Clean up code for older cmake versions

Freshen some of our CI build_.bash scripts.

Note that this will NOT be backported to the current release branch, but it will be part of the upcoming 1.13 family of releases.

[lgritz]

This has sat for a while without any comments. Last chance to object...

[lgritz]

No objections, so I'm merging this.