[Bug other/95005] New: zstd.h not found if installed in non-system prefix

[gcc at ikkoku dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95005

            Bug ID: 95005
           Summary: zstd.h not found if installed in non-system prefix
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: other
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc at ikkoku dot de
  Target Milestone: ---

Created attachment 48482
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48482&action=edit
Add zstd include directory

While adding GCC 10.1.0 to Spack, I ran into a problem with zstd: When
specifying a non-system prefix with --with-zstd (Spack installs each package in
its own prefix), the build of lto-compress.o failed because it could not find
the zstd.h header.

The attached patch fixes the problem. It should probably be changed to export
the appropriate compiler flag from configure.ac to avoid the ifeq in the
Makefile. However, I wanted to avoid touching configure.ac so we do not have to
regenerate configure.