Define HAVE_ for math long double functions declared in vxworks headers

When cross-building for vxworks, test for declarations of long double
functions in math.h.  We don't normally test for these functions when
cross compiling, because link tests don't work, or ever really, but
not defining them as available causes replacements to be defined in
ways that may cause duplicate definition linker errors if the units
defining both the replacement and the actual implementation are
brought in because of other symbols.


for libstdc++-v3/ChangeLog

	* crossconfig.m4 (GLIBCXX_CROSSCONFIG) [*-vxworks*]: Define
	long double functions as available if declared by math.h.
	(GLIBCXX_CHECK_MATH_DECL, GLIBCXX_CHECK_MATH_DECLS): New.
	* configure: Rebuild.

From-SVN: r279731