better check for libcif++

configure

@@ -6812,6 +6812,13 @@ fi
 # Check whether --with-cif++ was given.
 if test "${with_cif__+set}" = set; then :
   withval=$with_cif__;
+			if test -d ${withval}/include; then :
+
+else
+
+				as_fn_error $? "'${withval}'' is not a valid directory for --with-cif++" "$LINENO" 5
+
+fi
 
 			CIFPP_CFLAGS="-I ${withval}/include"
 			CIFPP_LIBS="-L${withval}/.libs -lcif++"

configure.ac

@@ -74,6 +74,9 @@ AC_ARG_WITH([cif++],
 	AS_HELP_STRING([--with-cif++=@<:@location@:>@],
 		[Use the cif++ library as specified.]),
 		[
+			AS_IF([test -d ${withval}/include], [], [
+				AC_MSG_ERROR(['${withval}'' is not a valid directory for --with-cif++])
+			])
 			dnl AC_SUBST([CIFPP_CFLAGS], ["-I ${withval}/include"])
 			dnl AC_SUBST([CIFPP_LIBS], ["-L${withval}/.libs -lcif++"])