cmake: build stage3 by default

This is a simplification of the cmake build script which introduces a
new "stage3" target that is built by default, which builds and installs
a stage3 zig.

It greatly simplifies the build instructions for Zig, making it conform
to the regular cmake routine, while still producing a stage3 artifact.

cmake/install.cmake

@@ -1,37 +0,0 @@
-message("-- Installing: ${CMAKE_INSTALL_PREFIX}/lib")
-
-if(NOT EXISTS ${zig_EXE})
-    message("::")
-    message(":: ERROR: Executable not found")
-    message(":: (execute_process)")
-    message("::")
-    message(":: executable: ${zig_EXE}")
-    message("::")
-    message(FATAL_ERROR)
-endif()
-
-execute_process(COMMAND ${zig_EXE} ${ZIG_INSTALL_ARGS}
-    WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
-    RESULT_VARIABLE _result
-)
-if(_result)
-    message("::")
-    message(":: ERROR: ${_result}")
-    message(":: (execute_process)")
-
-    string(REPLACE ";" " " s_INSTALL_LIBSTAGE2_ARGS "${ZIG_INSTALL_ARGS}")
-    message("::")
-    message(":: argv: ${zig_EXE} ${s_INSTALL_LIBSTAGE2_ARGS}")
-
-    set(_args ${zig_EXE} ${ZIG_INSTALL_ARGS})
-    list(LENGTH _args _len)
-    math(EXPR _len "${_len} - 1")
-    message("::")
-    foreach(_i RANGE 0 ${_len})
-        list(GET _args ${_i} _arg)
-        message(":: argv[${_i}]: ${_arg}")
-    endforeach()
-
-    message("::")
-    message(FATAL_ERROR)
-endif()