Linux: shell: bash: Makefile:5: *** missing separator. Stop.

如果从网上找这个的原因可能会找到一堆
https://til.linyiru.com/2018/04/make/missing-separator/

但是如果真的要看原因，就需要看make的文档或者是源代码。
比如源代码里有这么一段：

@item missing separator.  Stop.
@itemx missing separator (did you mean TAB instead of 8 spaces?).  Stop.
This means that @code{make} could not understand much of anything
about the makefile line it just read.  GNU @code{make} looks for
various separators (@code{:}, @code{=}, recipe prefix characters,
etc.) to indicate what kind of line it's parsing.  This message means
it couldn't find a valid one.

#: src/read.c:1154
#, c-format
msgid "missing separator (did you mean TAB instead of 8 spaces?)"
msgstr "分隔符缺失 (你大概想用 TAB，而不是八个空格)"

#: src/read.c:1156
#, c-format
msgid "missing separator"
msgstr "缺失分隔符"

源代码，如果一行的开始多几个空格，就可能导致make读到EOL。因为前几个都是无意义的字符：空格。

        /* If the word we're looking at is EOL, see if there's _anything_
           on the line.  If not, a variable expanded to nothing, so ignore
           it.  If so, we can't parse this line so punt.  */
        if (wtype == w_eol)
          {
            if (*p2 == '\0')
              continue;

            /* There's no need to be ivory-tower about this: check for
               one of the most common bugs found in makefiles...  */
            if (cmd_prefix == '\t' && strneq (line, "        ", 8))
              O (fatal, fstart, _("missing separator (did you mean TAB instead of 8 spaces?)"));
            else
              O (fatal, fstart, _("missing separator"));
          }