BLD: clean up incorrect-but-hardcoded define for strtold_l check.

GNU-specific and missing on many platforms, see
https://www.gnu.org/software///gnulib/manual/html_node/strtold_005fl.html

If it does go missing, the only problem seems to be some minor changes
in long double printing code, failing two tests:

```
 ______________________________ test_str_roundtrip ______________________________

      @pytest.mark.skipif(IS_MUSL,
                          reason="test flaky on musllinux")
      @pytest.mark.skipif(LD_INFO.precision + 2 >= repr_precision,
                          reason="repr precision not enough to show eps")
      def test_str_roundtrip():
          # We will only see eps in repr if within printing precision.
          o = 1 + LD_INFO.eps
  >       assert_equal(np.longdouble(str(o)), o, "str was %s" % str(o))
  E       AssertionError:
  E       Items are not equal: str was 1.0000000000000000001
  E        ACTUAL: np.longdouble('1.0')
  E        DESIRED: np.longdouble('1.0000000000000000001')

  o          = np.longdouble('1.0000000000000000001')

  numpy/_core/tests/test_longdouble.py:43: AssertionError
  ____________________ TestRealScalars.test_dragon4_interface ____________________

  self = <numpy._core.tests.test_scalarprint.TestRealScalars object at 0x7f5804791df0>

      def test_dragon4_interface(self):
          tps = [np.float16, np.float32, np.float64]
          # test is flaky for musllinux on np.float128
          if hasattr(np, 'float128') and not IS_MUSL:
              tps.append(np.float128)

          fpos = np.format_float_positional
          fsci = np.format_float_scientific

          for tp in tps:
              # test padding
              assert_equal(fpos(tp('1.0'), pad_left=4, pad_right=4), "   1.    ")
              assert_equal(fpos(tp('-1.0'), pad_left=4, pad_right=4), "  -1.    ")
  >           assert_equal(fpos(tp('-10.2'),
                           pad_left=4, pad_right=4), " -10.2   ")
  E           AssertionError:
  E           Items are not equal:
  E            ACTUAL: ' -10.1999999999999992895'
  E            DESIRED: ' -10.2   '

  fpos       = <function format_float_positional at 0x7f580b047700>
  fsci       = <function format_float_scientific at 0x7f580b047670>
  self       = <numpy._core.tests.test_scalarprint.TestRealScalars object at 0x7f5804791df0>
  tp         = <class 'numpy.longdouble'>
  tps        = [<class 'numpy.float16'>, <class 'numpy.float32'>, <class 'numpy.float64'>, <class 'numpy.longdouble'>]

  numpy/_core/tests/test_scalarprint.py:276: AssertionError
  =========================== short test summary info ============================
  FAILED numpy/_core/tests/test_longdouble.py::test_str_roundtrip - AssertionError:
  Items are not equal: str was 1.0000000000000000001
   ACTUAL: np.longdouble('1.0')
   DESIRED: np.longdouble('1.0000000000000000001')
  FAILED numpy/_core/tests/test_scalarprint.py::TestRealScalars::test_dragon4_interface - AssertionError:
  Items are not equal:
   ACTUAL: ' -10.1999999999999992895'
   DESIRED: ' -10.2   '
```

numpy/_core/meson.build

@@ -255,37 +255,6 @@ foreach filefunc_maybe: optional_file_funcs
   endif
 endforeach
 
-# Optional locale function
-have_strtold_l = cc.has_function('strtold_l', include_directories: inc_curdir,
-  prefix:'''
-  #include <stdlib.h>
-  #include <xlocale.h>
-  #include "feature_detection_locale.h"
-''')
-if not have_strtold_l
-  # Retry with locale.h, seems to vary across Linux distros
-  have_strtold_l = cc.has_function('strtold_l', include_directories: inc_curdir,
-    prefix:'''
-    #include <stdlib.h>
-    #include <locale.h>
-    #include "feature_detection_locale.h"
-  ''')
-endif
-if have_strtold_l
-  cdata.set10('HAVE_STRTOLD_L', true)
-else
-  # FIXME: this is wrong! the HAVE_ define should not exist, or it'll be
-  # interpreted as the function being available (true/false does nothing, see
-  # note on HAVE_ defines higher up). This is necessary though in order to make
-  # the Linux CI job pass. So either the check is wrong somehow, or this
-  # function is not available in CI. For the latter there is a fallback path,
-  # but that is broken because we don't have the exact long double
-  # representation checks.
-  if cc.get_argument_syntax() != 'msvc'
-    cdata.set10('HAVE_STRTOLD_L', false)
-  endif
-endif
-
 # Other optional functions
 optional_misc_funcs = [
   'backtrace',
@@ -303,7 +272,7 @@ endforeach
 # SSE headers only enabled automatically on amd64/x32 builds
 optional_headers = [
   'features.h',   # for glibc version linux
-  'xlocale.h',    # see GH#8367
+  'xlocale.h',    # removed in glibc 2.26, but may still be useful - see gh-8367
   'dlfcn.h',      # dladdr
   'execinfo.h',   # backtrace
   'libunwind.h',  # backtrace for LLVM/Clang using libunwind
@@ -315,6 +284,18 @@ foreach header: optional_headers
   endif
 endforeach
 
+# Optional locale function - GNU-specific
+_strtold_prefix = '''#define _GNU_SOURCE
+#include <locale.h>
+#include <stdlib.h>
+'''
+if cdata.get('HAVE_XLOCALE_H', 0) == 1
+  _strtold_prefix += '#include <xlocale.h>'
+endif
+if cc.has_function('strtold_l', include_directories: inc_curdir, prefix: _strtold_prefix)
+  cdata.set10('HAVE_STRTOLD_L', true)
+endif
+
 # Optional compiler attributes
 # TODO: this doesn't work with cc.has_function_attribute, see
 #       https://github.com/mesonbuild/meson/issues/10732

numpy/_core/src/common/numpyos.c

@@ -15,6 +15,7 @@
 #include <stdio.h>
 
 #ifdef HAVE_STRTOLD_L
+#define _GNU_SOURCE
 #include <stdlib.h>
 #ifdef HAVE_XLOCALE_H
 #include <xlocale.h>  // xlocale was removed in glibc 2.26, see gh-8367