Making gcc toolchain installs relocatable

[Keith Packard <keithp@keithp.com>]

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I submitted the referenced patch to extend the 'getenv' .specs function
back in August and didn't see any response, so I wanted to provide a bit
more context to see if that would help people understand why I wrote
this.

Here's a link to that message:

        https://gcc.gnu.org/pipermail/gcc-patches/2022-August/600452.html

I'm working with embedded toolchains where I want to distribute binary
versions of binutils, gcc and a suite of libraries in a tar file which
the user can unpack anywhere on their system. To make this work, I need
to create .spec file fragments that can locate the correct libraries
relative to the location where the toolchain was unpacked.

An easy way to do this, which doesn't depend on a default sysroot value,
is to use the GCC_EXEC_PREFIX environment variable in the .specs
file. Gcc sets that whenever it discovers that it hasn't been run from
the defined installation path. However, if the user does end up
installing gcc in the defined installation path, then that variable
isn't set at all. If a .specs file attempts to reference the variable,
gcc will emit a fatal error and exit.

This patch makes it possible for the .specs file fragment to provide the
default path as a fallback for a missing environment variable so that,
instead of exiting, the correct value will be substituted instead.

By doing this, I can create portable .specs file fragments which work
wherever the toolchain is installed.

This patch seemed like the least invasive approach to solving this
problem, but there are two other approaches that could work, and which
would make the .specs files simpler:

 1. Always set the GCC_EXEC_PREFIX environment variable, even if GCC
    is executed from the expected location.

 2. Make %R in .specs files expand to the appropriate value even if
    there is no sysroot defined.

I'd be happy to provide an implementation of either of those if that
would be more acceptable.

-- 
-keith

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