Fix isinf/isnan declaration conflict with C++11

GLIBC declares isinf and isnan as expected by Unix98 and for C99 programs
these functions are hidden by the generics inf and isnan macros.
However C++11 defines isinf and isnan with the same semantics as C99
but requires that they are functions not macros (C++11 26.8 [c.math]
paragraph 10).

This then results in a conflict for perfectly valid C++11 programs:

--
using std::isinf;
using std::isnan;

double d1 = isinf(1.0);
double d2 = isnan(1.0);

d.cc:3:12: error: ‘constexpr bool std::isinf(double)’ conflicts with a previous declaration
 using std::isinf;
[...]
/usr/include/bits/mathcalls.h:201:1: note: previous declaration ‘int isinf(double)’
 __MATHDECL_1 (int,isinf,, (_Mdouble_ __value)) __attribute__ ((__const__));
[...]
--

This patch fixes the prototypes by leaving the obsolete functions
defined for C++98 code (since they do not conflict with any standard
function in C++98), however preventing them on C++11.

No issues found in libstdc++ tests and check on x86_64 and i686 with
glibc testsuite.

Patch from Jonathan Wakely  <jwakely.gcc@gmail.com>.

	[BZ #19439]
	* math/bits/mathcalls.h
	[!__cplusplus || __cplusplus < 201103L] (isinf): Do not declare
	prototype.
	[!__cplusplus || __cplusplus < 201103L] (isnan): Likewise.

ChangeLog

@@ -1,3 +1,12 @@
+2016-01-11  Jonathan Wakely  <jwakely.gcc@gmail.com>
+	    Adhemerval Zanella  <adhemerval.zanella@linaro.org>
+
+	[BZ #19439]
+	* math/bits/mathcalls.h
+	[!__cplusplus || __cplusplus < 201103L] (isinf): Do not declare
+	prototype.
+	[!__cplusplus || __cplusplus < 201103L] (isnan): Likewise.
+
 2016-01-11  Andreas Schwab  <schwab@suse.de>
 
 	[BZ #19253]

math/bits/mathcalls.h

@@ -196,9 +196,11 @@ __MATHDECL_1 (int,__finite,, (_Mdouble_ __value)) __attribute__ ((__const__));
 _Mdouble_END_NAMESPACE
 
 #ifdef __USE_MISC
+# if !defined __cplusplus || __cplusplus < 201103L /* Conflicts with C++11.  */
 /* Return 0 if VALUE is finite or NaN, +1 if it
    is +Infinity, -1 if it is -Infinity.  */
 __MATHDECL_1 (int,isinf,, (_Mdouble_ __value)) __attribute__ ((__const__));
+# endif
 
 /* Return nonzero if VALUE is finite and not NaN.  */
 __MATHDECL_1 (int,finite,, (_Mdouble_ __value)) __attribute__ ((__const__));
@@ -230,8 +232,10 @@ __END_NAMESPACE_C99
 __MATHDECL_1 (int,__isnan,, (_Mdouble_ __value)) __attribute__ ((__const__));
 
 #if defined __USE_MISC || (defined __USE_XOPEN && !defined __USE_XOPEN2K)
+# if !defined __cplusplus || __cplusplus < 201103L /* Conflicts with C++11.  */
 /* Return nonzero if VALUE is not a number.  */
 __MATHDECL_1 (int,isnan,, (_Mdouble_ __value)) __attribute__ ((__const__));
+# endif
 #endif
 
 #if defined __USE_MISC || (defined __USE_XOPEN && __MATH_DECLARING_DOUBLE)