[Bug libstdc++/89023] New: libstdc++ test failure; can't find omp.h with --disable-libgomp

[sandra at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89023

            Bug ID: 89023
           Summary: libstdc++ test failure; can't find omp.h with
                    --disable-libgomp
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sandra at gcc dot gnu.org
  Target Milestone: ---

On nios2-elf, I'm seeing

FAIL: 28_regex/headers/regex/parallel_mode.cc (test for excess errors)

Specifically:

In file included from
/path/to/nios2-elf/include/c++/9.0.0/parallel/algobase.h:40,
                 from
/path/to/nios2-elf/include/c++/9.0.0/bits/stl_algobase.h:1459,
                 from /path/to/nios2-elf/include/c++/9.0.0/algorithm:61,
                 from /path/to/nios2-elf/include/c++/9.0.0/regex:38,
                 from
/path/to/src/gcc/libstdc++-v3/testsuite/28_regex/headers/regex/parallel_mode.cc:22:
/path/to/nios2-elf/include/c++/9.0.0/parallel/base.h:37: fatal error: omp.h: No
such file or directory

I'm building for this bare-metal target with --disable-libgomp so of course
there is no omp.h being installed, and this test cannot work.

Is there some reason this test is conditionalized differently than, say, 
28_regex/headers/regex/parallel_mode.cc ??  That one is properly recognized as
unsupported on this target.

Also, the underlying failure seems related to pr35887.  Why is libstdc++ even
installing headers that depend on omp.h with --disable-libgomp?