feature tests: signbit detection fix for ia64

An Intel Itanium fix from OpenSUSE:
https://build.opensuse.org/package/view_file/shells/ksh/ksh93-ia64.dif

Author: McDutchie

src/cmd/ksh93/features/math.sh

@@ -135,6 +135,18 @@ echo "#include "
 case $_hdr_ieeefp in
 1)	echo "#include " ;;
 esac
+cat <<!
+#if defined(__ia64__) && defined(signbit)
+# if defined __GNUC__ && __GNUC__ >= 4
+#  define __signbitl(f)		__builtin_signbitl(f)
+# else
+#  include 
+#  if _lib_copysignl
+#   define __signbitl(f)	(int)(copysignl(1.0,(f))<0.0)
+#  endif
+# endif
+#endif
+!
 echo
 
 : generate the intercept functions and table entries

src/lib/libast/sfio/sfcvt.c

@@ -55,6 +55,16 @@ static char		*Zero = "0";
 #endif
 #endif
 
+#if defined(__ia64__) && defined(signbit)
+# if defined __GNUC__ && __GNUC__ >= 4
+#  define __signbitl(f)			__builtin_signbitl(f)
+# else
+#  if _lib_copysignl
+#    define __signbitl(f)	(int)(copysignl(1.0,(f))<0.0)
+#  endif
+# endif
+#endif
+
 #if !_lib_signbit
 #if !_ast_fltmax_double
 static int neg0ld(Sfdouble_t f)