[gcc r11-6259] libstdc++: Fix build failure due to missing <langinfo.h> [PR98374]

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:d7bab388b818fc21dbb9111311e114ae33e11fff

commit r11-6259-gd7bab388b818fc21dbb9111311e114ae33e11fff
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Dec 18 11:52:17 2020 -0500

    libstdc++: Fix build failure due to missing <langinfo.h> [PR98374]
    
    This should fix a build failure on Windows which lacks <langinfo.h>,
    from which we use nl_langinfo() to obtain the radix character of the
    current locale.  (We can't use the more portable localeconv() from
    <clocale> to obtain the radix character of the current locale here
    because it's not thread-safe, unfortunately.)
    
    This change means that on Windows and other such platforms, we'll just
    always assume the radix character used by printf is '.' when formatting
    a long double through it.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/98374
            * src/c++17/floating_to_chars.cc: Guard include of <langinfo.h>
            with __has_include.
            (__floating_to_chars_precision) [!defined(RADIXCHAR)]: Don't
            attempt to obtain the radix character of the current locale,
            just assume it's '.'.

Diff:
---
 libstdc++-v3/src/c++17/floating_to_chars.cc | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/src/c++17/floating_to_chars.cc b/libstdc++-v3/src/c++17/floating_to_chars.cc
index 474e791e717..6470fbb0b95 100644
--- a/libstdc++-v3/src/c++17/floating_to_chars.cc
+++ b/libstdc++-v3/src/c++17/floating_to_chars.cc
@@ -33,7 +33,9 @@
 #include <cmath>
 #include <cstdio>
 #include <cstring>
-#include <langinfo.h>
+#if __has_include(<langinfo.h>)
+# include <langinfo.h> // for nl_langinfo
+#endif
 #include <optional>
 #include <string_view>
 #include <type_traits>
@@ -1113,6 +1115,7 @@ template<typename T>
 	// to handle a radix point that's different from '.'.
 	char radix[6] = {'.', '\0', '\0', '\0', '\0', '\0'};
 	if (effective_precision > 0)
+#ifdef RADIXCHAR
 	  // ???: Can nl_langinfo() ever return null?
 	  if (const char* const radix_ptr = nl_langinfo(RADIXCHAR))
 	    {
@@ -1121,6 +1124,7 @@ template<typename T>
 	      // UTF-8 character) wide.
 	      __glibcxx_assert(radix[4] == '\0');
 	    }
+#endif
 
 	// Compute straightforward upper bounds on the output length.
 	int output_length_upper_bound;