prevent "undef var" errors on gcc --help or --version

[Olivier Hainque]

Hello,

self-specs setup with configure --with-specs are allowed to contain %:getenv
environment variable references. We are using this capability in a few cases
for cross ports.

In such configurations, gcc --help or gcc --version fail if the variables
aren't defined, like so:

gcc --version             
Using built-in specs.
gcc: fatal error: environment variable ‘<VARIABLE_NAME_HERE>’ not defined
compilation terminated.

It is a bit annoying to have to define environment variables just to
be able to retrieve version info. We have been using a close variant of
the attached patch to address this on gcc-4.7 and 4.9 for a while now.

The patch was adjusted for mainline, I tested that it still operates
as intended and performed a regular bootstrap + testsuite cycle showing
no regression on x86_64-linux.

OK to commit ?

Thanks much in advance for your feedback,

With Kind Regards,

Olivier


2016-01-08  Olivier Hainque  <hainque@adacore.com>

	* gcc.c (spec_undefvar_allowed): New global.
       (process_command): Set to true when running for --version or --help
       alone, or together.
       (getenv_spec_function): When the variable is not defined, use the
       variable name as the variable value if we're allowed not to issue
       a fatal error.

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