Porting the module 'fenv-environment'

[Bruno Haible]

Since the module 'fenv-environment' is likely to need workarounds on
future platforms, here is the best procedure to do so. It works even on
proprietary systems like macOS, AIX, MSVC.

1) Prepare a testdir of the relevant modules:

  $ ./gnulib-tool --create-testdir --dir=../testdir4 --single-configure \
                  fenv-rounding \
                  fenv-exceptions-tracking-c23 \
                  fenv-exceptions-state-c23 \
                  fenv-exceptions-trapping \
                  fenv-environment

2) Compile it on any platform, with configure option -C, to see which
   autoconf cache variables are available:

  $ ./configure -C; grep gl_cv_func_fe config.cache
  gl_cv_func_feenableexcept_in_libm=${gl_cv_func_feenableexcept_in_libm=yes}
  gl_cv_func_feenableexcept_no_libm=${gl_cv_func_feenableexcept_no_libm=no}
  gl_cv_func_fegetenv_works=${gl_cv_func_fegetenv_works='guessing yes'}
  gl_cv_func_feholdexcept_works=${gl_cv_func_feholdexcept_works='guessing yes'}
  gl_cv_func_feraiseexcept_in_libm=${gl_cv_func_feraiseexcept_in_libm=yes}
  gl_cv_func_feraiseexcept_no_libm=${gl_cv_func_feraiseexcept_no_libm=no}
  gl_cv_func_fesetenv_in_libm=${gl_cv_func_fesetenv_in_libm=yes}
  gl_cv_func_fesetenv_no_libm=${gl_cv_func_fesetenv_no_libm=no}
  gl_cv_func_fesetenv_works=${gl_cv_func_fesetenv_works='guessing yes'}
  gl_cv_func_fesetexcept_in_libm=${gl_cv_func_fesetexcept_in_libm=yes}
  gl_cv_func_fesetexcept_no_libm=${gl_cv_func_fesetexcept_no_libm=no}
  gl_cv_func_fesetexcept_works=${gl_cv_func_fesetexcept_works=yes}
  gl_cv_func_fesetexceptflag_in_libm=${gl_cv_func_fesetexceptflag_in_libm=yes}
  gl_cv_func_fesetexceptflag_no_libm=${gl_cv_func_fesetexceptflag_no_libm=no}
  gl_cv_func_fesetexceptflag_works1=${gl_cv_func_fesetexceptflag_works1=no}
  gl_cv_func_fesetexceptflag_works2=${gl_cv_func_fesetexceptflag_works2=yes}
  gl_cv_func_fesetround_in_libm=${gl_cv_func_fesetround_in_libm=yes}
  gl_cv_func_fesetround_no_libm=${gl_cv_func_fesetround_no_libm=no}
  gl_cv_func_fesetround_works=${gl_cv_func_fesetround_works='guessing yes'}
  gl_cv_func_fetestexceptflag_in_libm=${gl_cv_func_fetestexceptflag_in_libm=yes}
  gl_cv_func_fetestexceptflag_no_libm=${gl_cv_func_fetestexceptflag_no_libm=no}
  gl_cv_func_feupdateenv_works=${gl_cv_func_feupdateenv_works='guessing yes'}

  Here, the most useful among these variables are the *_works* variables:

  gl_cv_func_fegetenv_works
  gl_cv_func_feholdexcept_works
  gl_cv_func_fesetenv_works
  gl_cv_func_feupdateenv_works

3) Build the testdir on the target platform, and look at the test failures.

4) If there are test failures in other modules, that is, in the
   fenv-rounding tests, fenv-exceptions-tracking* tests, or
   fenv-exceptions-trapping tests, fix them first.
   This is necessary because the fenv-environment tests depend on
   fenv-rounding, fenv-exceptions-tracking*, and fenv-exceptions-trapping.

5) Find the minimal set of
     gl_cv_func_fegetenv_works=no
     gl_cv_func_feholdexcept_works=no
     gl_cv_func_fesetenv_works=no
     gl_cv_func_feupdateenv_works=no
   assignments such that all unit tests pass.

   Start with all assignments together:
     make distclean; \
     gl_cv_func_fegetenv_works=no \
     gl_cv_func_feholdexcept_works=no \
     gl_cv_func_fesetenv_works=no \
     gl_cv_func_feupdateenv_works=no \
     ./configure -C && make && make check

   Note that because fegetenv and fesetenv are currently implemented in the
   same compilation unit, gl_cv_func_fegetenv_works=no and
   gl_cv_func_fesetenv_works=no are currently equivalent. That is, you need
   only one of these two; the other one is implied.

6) When you have found this minimal set, then for each of these assignments:
     - Reconfigure with this one =yes instead of =no.
     - Look at the test failures. This tells you what is wrong with that
       function.

Bruno