(autoconf.info)Special Chars in Names

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11.3 Special Characters in Make Macro Names

Posix limits macro names to nonempty strings containing only ASCII
letters and digits, `.', and `_'.  Many `make' implementations allow a
wider variety of characters, but portable makefiles should avoid them.
It is portable to start a name with a special character, e.g.,

   Some ancient `make' implementations don't support leading
underscores in macro names.  An example is NEWS-OS 4.2R.

     $ cat Makefile
     _am_include = #
     _am_quote =
     all:; @echo this is test
     $ make
     Make: Must be a separator on rules line 2.  Stop.
     $ cat Makefile2
     am_include = #
     am_quote =
     all:; @echo this is test
     $ make -f Makefile2
     this is test

However, this problem is no longer of practical concern.

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