When specifying a non-system prefix with --with-zstd, the build fails because the...

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.

gcc/
	* Makefile.in (ZSTD_INC): Define.
	(ZSTD_LIB): Include ZSTD_LDFLAGS.
	(CFLAGS-lto-compress.o): Add ZSTD_INC.
	* configure.ac (ZSTD_CPPFLAGS, ZSTD_LDFLAGS): New variables for
	AC_SUBST.
	* configure: Rebuilt.