Migrate to cmake

[luke-jr]

Bitcoin Core 29.x is switching to CMake instead of Autotools, which means everything touching the build system will need rewriting.

This includes:

• sys_leveldb
• sys_libsecp256k1
• bugfix_asm_pragmas
• 2020mingwthrd (should be evaluated if we can just drop it entirely?)
• boost_171_177_workarounds
• hww_windows
• mempressure
• conf_append_cxxflags
• ionice + ionice_win
• dsha256_power8
• tor_subprocess
• win_taskbar_progress
• macos_dmg
• svg_icon
• tbc_font
• knots_branding
• subproc_closefds (run_command: Close non-std fds when execing slave processes bitcoin/bitcoin#30756)

Additionally, this removes the major pre-generation step of the source tarball, so it may make sense to just require rsvg-tools and ImageMagick rather than try to pre-render the icons.

(If anyone would like to work on these, ping me to ensure you have the latest branches.)