Integrate w/ sofort's ${NATIVE_{C,CXX,LD}FLAGS}.
    
    groups/{000.host_tools,010.host_deps,200.native_packages}.group:${*_SOFORT_NATIVE_C{,XX}FLAGS}: build w/ -I${PREFIX}/include.
    groups/010.host_deps.group:python[23]_host:${PKG_ENV_VARS_EXTRA}: updated.
    groups/010.host_deps.group:python[23]_host:${PKG_SOFORT_FORCE}: force calling ex_pkg_env_sofort().
    groups/010.host_deps.group:python3_host:${PKG_SOFORT_NATIVE_CC}: build w/ ${DEFAULT_HOST_CC} (via midipix.)
    groups/221.native_packages_dev.group:python[23]:${PKG_SOFORT_FORCE}: force calling ex_pkg_env_sofort().
    groups/221.native_packages_dev.group:${PKG_{ENV_VARS_EXTRA,SOFORT_NATIVE_CFLAGS_EXTRA}}: updated.
    midipix.env:${DEFAULT_BUILD_VARS}: adds SOFORT_NATIVE_{{CC,CXX,LD},{C,CXX,LD}FLAGS{,_EXTRA}}.
    patches/python3_host-3.6.3.local.patch: removes obsolete patch.
    subr/ex_pkg_env.subr:ex_pkg_env_sofort(): initial implementation.
    subr/ex_pkg_env.subr:ex_pkg_env(): call ex_pkg_env_sofort() initially.
    subr/pkg_configure_autotools.subr: call ex_pkg_env_sofort().