zstd not found if installed in non-system prefix

[Matthias Klose]

On 5/20/20 9:32 PM, Michael Kuhn wrote:
> Hi,
> 
> when specifying a non-system prefix with --with-zstd, the build fails
> because the header and library cannot be found (see 
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95005).
> 
> The attached patch fixes the problem and is what we use in Spack to
> make GCC build with zstd support.

documentation for this is missing in gcc/doc/install.texi.  Also please don't
introduce configurations via environment variables, but add options like for
gmp. See

@item --with-gmp=@var{pathname}
@itemx --with-gmp-include=@var{pathname}
@itemx --with-gmp-lib=@var{pathname}

Matthias