libstdc++: Check _GLIBCXX_USE_C99_STDLIB for strtof and strtold

On broken systems we only have strtod, not strtof and strtold. Just use
strtod for all types, even though that will produce incorrect results in
some cases.

Similarly, if _GLIBCXX_USE_C99_MATH is not defined then std::isinf won't
be declared. Just refer to it unqualified, which should find the C
library's isinf macro if that hasn't been #undef'd by <cmath>.

libstdc++-v3/ChangeLog:

	* src/c++17/floating_from_chars.cc (from_chars_impl): Use
	isinf unqualified.
	[!_GLIBCXX_USE_C99_STDLIB]: Use strtod for float and long
	double.

1 files changed, 6 insertions, 2 deletions

diff --git a/libstdc++-v3/src/c++17/floating_from_chars.cc b/libstdc++-v3/src/c++17/floating_from_chars.cc
index f1519e5..26b69a3 100644
--- a/libstdc++-v3/src/c++17/floating_from_chars.cc
+++ b/libstdc++-v3/src/c++17/floating_from_chars.cc
@@ -300,12 +300,16 @@ namespace
 	errno = 0;
 	char* endptr;
 	T tmpval;
+#if _GLIBCXX_USE_C99_STDLIB
 	if constexpr (is_same_v<T, float>)
 	  tmpval = std::strtof(str, &endptr);
-	if constexpr (is_same_v<T, double>)
+	else if constexpr (is_same_v<T, double>)
 	  tmpval = std::strtod(str, &endptr);
 	else if constexpr (is_same_v<T, long double>)
 	  tmpval = std::strtold(str, &endptr);
+#else
+	tmpval = std::strtod(str, &endptr);
+#endif
 	const int conv_errno = std::__exchange(errno, save_errno);
 
 #if _GLIBCXX_USE_C99_FENV_TR1
@@ -319,7 +323,7 @@ namespace
 	const ptrdiff_t n = endptr - str;
 	if (conv_errno == ERANGE) [[unlikely]]
 	  {
-	    if (std::isinf(tmpval)) // overflow
+	    if (isinf(tmpval)) // overflow
 	      ec = errc::result_out_of_range;
 	    else // underflow (LWG 3081 wants to set value = tmpval here)
 	      ec = errc::result_out_of_range;