cmake: separate installing and building (again)

cmake/install.cmake

@@ -0,0 +1,24 @@
+set(ZIG_INSTALL_ARGS ${ZIG_BUILD_ARGS} --prefix "${CMAKE_INSTALL_PREFIX}")
+execute_process(COMMAND "${ZIG_EXECUTABLE}" ${ZIG_INSTALL_ARGS} WORKING_DIRECTORY "${CMAKE_SOURCE_DIR}" RESULT_VARIABLE _result)
+
+if(_result)
+    message("::")
+    message(":: ERROR: ${_result}")
+    message(":: (execute_process)")
+
+    list(JOIN ZIG_INSTALL_ARGS " " s_INSTALL_LIBSTAGE2_ARGS)
+    message("::")
+    message(":: argv: ${ZIG_EXECUTABLE} ${s_INSTALL_LIBSTAGE2_ARGS}")
+
+    set(_args ${ZIG_EXECUTABLE} ${ZIG_INSTALL_ARGS})
+    list(LENGTH _args _len)
+    math(EXPR _len "${_len} - 1")
+    message("::")
+    foreach(_i RANGE 0 ${_len})
+        list(GET _args ${_i} _arg)
+        message(":: argv[${_i}]: ${_arg}")
+    endforeach()
+
+    message("::")
+    message(FATAL_ERROR)
+endif()