[Bug libstdc++/69413] New: [6 Regression] r232327 prevents libstdc++ working after upgrading glibc to 2.23

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69413

            Bug ID: 69413
           Summary: [6 Regression] r232327 prevents libstdc++ working
                    after upgrading glibc to 2.23
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

The fixes for PR 48891 require libstdc++ to be rebuilt after upgrading glibc to
current trunk (which will be 2.23), because the configure-time checks for isinf
and isnan give the wrong answer for the new glibc.

There needs to be a compile-time check so that rebuilding gcc isn't needed (or
glibc needs to define a macro telling us the isinf and isnan declarations have
been suppressed).