libstdc++: testsuite: complex proj requirements

The template version of complex::proj returns its argument without
testing for infinities, and that's all we have when neither C99
complex nor C99 math functions are available, and it seems too hard to
do better without isinf and copysign.

I suppose just calling them and expecting users will supply
specializations as needed has been ruled out, and so has refraining
from defining it when it can't be implemented correctly.

It's pointless to run the proj.cc test under these circumstances, so
arrange for it to be skipped.  In an unusual way, after trying to
introduce dg-require tests for ccomplex-or-cmath, and found their
results to be misleading due to variations across -std=* versions.


for  libstdc++-v3/ChangeLog

	* testsuite/26_numerics/complex/proj.cc: Skip test in the
	circumstances in which the implementation of proj is known to
	be broken.

libstdc++-v3/testsuite/26_numerics/complex/proj.cc

@@ -397,6 +397,19 @@ test03()
 int
 main()
 {
+  /* If neither of these macros is nonzero, proj calls a
+     specialization of the __complex_proj template, that just returns
+     its argument, without testing for infinities, rendering the whole
+     test pointless, and failing (as intended/noted in the
+     implementation) the cases that involve infinities.  Alas, the
+     normal ways to skip tests may not work: we don't have a test for
+     C99_COMPLEX, and these macros may vary depending on -std=*, but
+     macro tests wouldn't take them into account.  */
+#if ! (_GLIBCXX_USE_C99_COMPLEX || _GLIBCXX_USE_C99_MATH_TR1)
+  if (true)
+    return 0;
+#endif
+
   test01();
   test02();
   test03();