* acinclude.m4 (GCC_GLIBC_VERSION_GTE_IFELSE): New configure
macro.
* configure.ac: Determine target_header_dir earlier.
(--with-glibc-version): New configure option.
Use GCC_GLIBC_VERSION_GTE_IFELSE in enable_gnu_unique_object,
gcc_cv_libc_provides_ssp and gcc_cv_target_ldbl128 tests.
* configure: Regenerate.
* doc/install.texi (--enable-gnu-unique-object): Don't refer to
native toolchains for default.
(--with-glibc-version): Document.
git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@204841 138bc75d-0d04-0410-961f-82ee72b054a4