Add configure-time test for log1p disrespecting zero sign.

configure.in

@@ -3372,6 +3372,10 @@ fi
 LIBS_SAVE=$LIBS
 LIBS="$LIBS $LIBM"
 
+AC_CHECK_FUNCS([acosh asinh atanh copysign erf erfc expm1 finite gamma])
+AC_CHECK_FUNCS([hypot lgamma log1p round tgamma])
+AC_CHECK_DECLS([isinf, isnan, isfinite], [], [], [[#include <math.h>]])
+
 # On FreeBSD 6.2, it appears that tanh(-0.) returns 0. instead of
 # -0. on some architectures.
 AC_MSG_CHECKING(whether tanh preserves the sign of zero)
@@ -3398,9 +3402,34 @@ then
   [Define if tanh(-0.) is -0., or if platform doesn't have signed zeros])
 fi
 
-AC_CHECK_FUNCS([acosh asinh atanh copysign erf erfc expm1 finite gamma])
-AC_CHECK_FUNCS([hypot lgamma log1p round tgamma])
-AC_CHECK_DECLS([isinf, isnan, isfinite], [], [], [[#include <math.h>]])
+if test "$ac_cv_func_log1p" = yes
+then
+    # On some versions of AIX, log1p(-0.) returns 0. instead of
+    # -0.  See issue #9920.
+    AC_MSG_CHECKING(whether log1p drops the sign of negative zero)
+    AC_CACHE_VAL(ac_cv_log1p_drops_zero_sign, [
+    AC_RUN_IFELSE([AC_LANG_SOURCE([[
+    #include <math.h>
+    #include <stdlib.h>
+    int main() {
+        /* Fail if the signs of log1p(-0.) and -0. can be
+	   distinguished. */
+        if (atan2(log1p(-0.), -1.) == atan2(-0., -1.))
+            return 0;
+        else
+            return 1;
+    }
+    ]])],
+    [ac_cv_log1p_drops_zero_sign=no],
+    [ac_cv_log1p_drops_zero_sign=yes],
+    [ac_cv_log1p_drops_zero_sign=no])])
+    AC_MSG_RESULT($ac_cv_log1p_drops_zero_sign)
+fi
+if test "$ac_cv_log1p_drops_zero_sign" = yes
+then
+  AC_DEFINE(LOG1P_DROPS_ZERO_SIGN, 1,
+  [Define if log1p(-0.) is 0. rather than -0.])
+fi
 
 LIBS=$LIBS_SAVE