Implements CMake support.
    
    etc/README.md: updated.
    groups/{000.host_tools,010.host_deps,200.native_packages,800.minipix}.group:${*_SOFORT_NATIVE_{C,CXX,LD}FLAGS}: merged into midipix.env.
    groups/010.host_deps.group:{clang,cmake_host,libzip,llvm}_host:${PKG_CMAKE_ARGS_EXTRA}: added.
    groups/010.host_deps.group:cmake_host:${PKG_CONFIGURE_TYPE}: configure w/ cmake.
    groups/010.host_deps.group:cmake_host:${PKG_INSTALL_FILES}: added.
    groups/010.host_deps.group:expat_host:${PKG_CONFIGURE_TYPE}: configure w/ autotools.
    groups/010.host_deps.group:python[23]_host:${PKG_{ENV_VARS_EXTRA,SOFORT_FORCE}}: removed.
    groups/010.host_deps.group:python[23]_host:${PKG_CONFIGURE_TYPE}: configure w/ sofort.
    groups/010.host_deps.group:python[23]_host:${PKG_SOFORT_NATIVE_CC}: removed.
    groups/221.native_packages_dev.group:python[23]:${PKG_SOFORT_FORCE}: removed.
    groups/221.native_packages_dev.group:python[23]:${PKG_CONFIGURE_TYPE}: configure w/ sofort.
    groups/231.native_packages_etc.group:{cmake,minizip,openjpeg,tdnf,xpdf}:${PKG_CMAKE_ARGS_EXTRA}: added.
    groups/231.native_packages_etc.group:cmake:${PKG_CONFIGURE_ARGS}: removed.
    groups/231.native_packages_etc.group:utf8proc:${PKG_IN_TREE}: don't build in-tree.
    groups/251.native_packages_lib.group:lib{jpeg_turbo,solv,ssh,zip}:${PKG_CMAKE_ARGS_EXTRA}: added.
    midipix.env:${DEFAULT_BUILD_VARS}: adds ${PKG_{CMAKE{,_{ARGS{,_EXTRA},LISTFILE}},CONFIGURE_TYPE},SOFORT_NATIVE_{C{C,XX},{C,CXX,LD}FLAGS,LD}}.
    midipix.env:${DEFAULT_BUILD_VARS}: removes ${PKG_SOFORT_FORCE}.
    midipix.env:${DEFAULT_{CMAKE_LISTFILE,{CROSS,HOST,NATIVE}_CMAKE}}: added.
    subr.rtl/rtl_complex.subr:rtl_export_vars(): ignore values during -u.
    subr.rtl/rtl_platform.subr:rtl_set_vars(): updated.
    subr/ex_pkg_env.subr:exp_pkg_env_set(): always convert ${PKG_{AR,CC,CXX,PKG_CONFIG,RANLIB}} to absolute pathnames.
    subr/ex_pkg_env.subr:ex_pkg_env_sofort(): merged into subr/pkg_configure.subr.
    subr/ex_pkg_restart.subr:ex_pkg_process_restart_spec(): remove extraneous echo.
    subr/pkg_configure.subr:pkgp_configure_{cmake,sofort}(): initial implementation.
    subr/pkg_configure.subr:pkg_configure(): attempt configuration using autotools, sofort, or cmake.
    subr/pkg_configure_autotools.subr: skip if ${PKG_CONFIGURE_TYPE} is set and not "autotools".
    subr/pkg_fetch_clean.subr: fix typo.
    vars/{clang_host,cmake{,_host},lib{jpeg_turbo,solv,ssh,zip{,_host}},llvm_host,lolcode,minizip,openjpeg,tdnf,xpdf}.vars: deleted.