Fix issue #1023 (#1024)

Author: mborland

include/boost/math/concepts/real_concept.hpp

@@ -45,8 +45,12 @@
 #  include <cstdio>
 #endif
 
-#if __has_include(<stdfloat>)
-#  include <stdfloat>
+#if defined __has_include
+#  if __cplusplus > 202002L || _MSVC_LANG > 202002L 
+#    if __has_include (<stdfloat>)
+#    include <stdfloat>
+#    endif
+#  endif
 #endif
 
 namespace boost{ namespace math{

include/boost/math/tools/config.hpp

@@ -152,11 +152,11 @@
      // libstdc++3 only defines to/from_chars for std::float128_t when one of these defines are set
      // otherwise we're right out of luck...
 #    if defined(_GLIBCXX_LDOUBLE_IS_IEEE_BINARY128) || defined(_GLIBCXX_HAVE_FLOAT128_MATH)
-#    include <cstring> // std::strlen is used with from_chars
-#    include <charconv>
-#    include <stdfloat>
-#    define BOOST_MATH_USE_CHARCONV_FOR_CONVERSION
-#endif
+#      include <cstring> // std::strlen is used with from_chars
+#      include <charconv>
+#      include <stdfloat>
+#      define BOOST_MATH_USE_CHARCONV_FOR_CONVERSION
+#    endif
 #  endif
 #endif
 

include/boost/math/tools/promotion.hpp

@@ -27,8 +27,10 @@
 #include <type_traits>
 
 #if defined __has_include
-#  if __has_include (<stdfloat>)
+#  if __cplusplus > 202002L || _MSVC_LANG > 202002L 
+#    if __has_include (<stdfloat>)
 #    include <stdfloat>
+#    endif
 #  endif
 #endif