libstdc++: testsuite: Guard use of C99 std::log2

This prevents the test from failing if the only thing not supported is
the text printed to the log about the size of the floating-point type.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/from_chars/4.cc: Only use log2 if C99 math
	functions are available.

libstdc++-v3/testsuite/20_util/from_chars/4.cc

@@ -300,9 +300,11 @@ test_max_mantissa()
 
   if (Float_limits::is_iec559 && Float_limits::digits < UInt_limits::digits)
   {
+#ifdef _GLIBCXX_USE_C99_MATH_TR1
     std::printf("Testing %d-bit float, using %zu-bit integer\n",
 	Float_limits::digits + (int)std::log2(Float_limits::max_exponent) + 1,
 	sizeof(UIntT) * __CHAR_BIT__);
+#endif
 
     std::from_chars_result res;
     FloatT flt;