[PATCH, Makefile] improve libsubdir variable transmission to sub-makes on Windows

[Olivier Hainque <hainque at adacore dot com>]

Hello,

To convey the value of libsubdir to sub-makes, gcc/Makefile.in has

  # Flags to pass to recursive makes.
  # CC is set by configure.
  # ??? The choices here will need some experimenting with.

  export AR_FOR_TARGET
  ...
  export libsubdir

then:

  FLAGS_TO_PASS = \
  ...
  (libsubdir not here)

This is not working well on cygwin environments where environment
variable names are translated to uppercase (so sub-makes evaluating
the variable with the lowercase name don't get the value).

The attached patch is a suggestion to address this by simply adding
"libsubdir" to the list of explicit FLAGS_TO_PASS.

Used in-house for a while, with gcc-6 on a wide variety of host/target
configurations.

Bootstrapped and regression tested with mainline on x86_64-linux.

OK to commit ?

Thanks in advance,

With Kind Regards,

Olivier

2017-09-01  Jerome Lambourg  <lambourg@adacore.com>

	* Makefile.in (FLAGS_TO_PASS): Add libsubdir.

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