isinf and isnan are macros, not functions; fix configure script
to use AC_CHECK_DECLS instead of AC_CHECK_FUNCS for these.
(See discussion in issue #4506)
diff --git a/configure.in b/configure.in
index ea8727c..e1854a2 100644
--- a/configure.in
+++ b/configure.in
@@ -3209,7 +3209,8 @@
AC_REPLACE_FUNCS(hypot)
-AC_CHECK_FUNCS(acosh asinh atanh copysign expm1 finite isinf isnan log1p)
+AC_CHECK_FUNCS([acosh asinh atanh copysign expm1 finite log1p])
+AC_CHECK_DECLS([isinf, isnan, isfinite], [], [], [[#include <math.h>]])
LIBS=$LIBS_SAVE