[Bug bootstrap/122459] New: --with-gxx-libcxx-include-dir is undocumented

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122459

            Bug ID: 122459
           Summary: --with-gxx-libcxx-include-dir is undocumented
           Product: gcc
           Version: 15.2.1
            Status: UNCONFIRMED
          Keywords: documentation
          Severity: minor
          Priority: P3
         Component: bootstrap
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
                CC: iains at gcc dot gnu.org
  Target Milestone: ---

https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Dialect-Options.html#index-stdlib
mentions that support for -stdlib needs to be enabled during configure, but we
don't document how to do that

Also at https://gcc.gnu.org/install/configure.html the --with-gxx-include-dir
and --with-specs options are documented under "The following standard autoconf
options are supported" but I'm pretty sure they're gcc-specific and not
standard autoconf options.

Maybe we should have a new subsection that documents 

--with-specs
--with-gxx-include-dir
--with-gxx-libcxx-include-dir