Prevent redefinition of WCHAR_MAX from testsuite/gcc.dg/cpp/ucs.c

gcc/testsuite/gcc.dg/cpp/ucs.c #include <limits.h>
and then crafts a definition of WCHAR_MAX depending
on __WCHAR_TYPE__.

The test fails in VxWorks configurations because WCHAR_MAX
is already exposed by the system limits.h.

The patch simply guards the tentative definition
by a check verifying if the macro is defined already, so
we're using the value exposed by limits.h in this case.

2019-12-30  Olivier Hainque  <hainque@adacore.com>

	* testsuite/gcc.dg/cpp/ucs.c: Prevent redefinition
	of WCHAR_MAX if already exposed by limits.h.

From-SVN: r279795