Omit test-math-isinff when no C++ compiler.

ChangeLog

@@ -1,5 +1,8 @@
 2016-03-04  Roland McGrath  <roland@hack.frob.com>
 
+	* math/Makefile (tests): Add test-math-isinff only if $(CXX) is
+	nonempty.
+
 	* Makefile ($(objpfx)c++-types-check.out): Fix conditionalization
 	to test for empty $(CXX) rather than $(CXX) of "no".
 

math/Makefile

@@ -114,7 +114,6 @@ tests = test-matherr test-fenv atest-exp atest-sincos atest-exp2 basic-test \
 	test-nearbyint-except-2 test-signgam-uchar test-signgam-uchar-init \
 	test-signgam-uint test-signgam-uint-init test-signgam-ullong \
 	test-signgam-ullong-init test-nan-overflow test-nan-payload \
-	test-math-isinff \
 	$(tests-static)
 tests-static = test-fpucw-static test-fpucw-ieee-static \
 	       test-signgam-uchar-static test-signgam-uchar-init-static \
@@ -124,6 +123,10 @@ tests-static = test-fpucw-static test-fpucw-ieee-static \
 # distinct from `double'.
 test-longdouble-yes = test-ldouble test-ildoubl test-ldouble-finite
 
+ifneq (,$(CXX))
+tests += test-math-isinff
+endif
+
 ifneq (no,$(PERL))
 libm-vec-tests = $(addprefix test-,$(libmvec-tests))
 libm-tests = test-float test-double $(test-longdouble-$(long-double-fcts)) \