Allow _GLIBCXX_HAVE_OBSOLETE_ISINF_ISNAN to be overridden

PR libstdc++/69413
	* config/os/gnu-linux/os_defines.h: Define
	_GLIBCXX_NO_OBSOLETE_ISINF_ISNAN_DYNAMIC.
	* include/c_global/cmath (isinf, isnan): Check it.
	* doc/xml/manual/internals.xml: Document it.
	* doc/html/*: Regenerate.

From-SVN: r232726

libstdc++-v3/ChangeLog

@@ -1,3 +1,12 @@
+2016-01-22  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/69413
+	* config/os/gnu-linux/os_defines.h: Define
+	_GLIBCXX_NO_OBSOLETE_ISINF_ISNAN_DYNAMIC.
+	* include/c_global/cmath (isinf, isnan): Check it.
+	* doc/xml/manual/internals.xml: Document it.
+	* doc/html/*: Regenerate.
+
 2016-01-21  Jonathan Wakely  <jwakely@redhat.com>
 
 	PR libstdc++/69406

libstdc++-v3/config/os/gnu-linux/os_defines.h

@@ -45,4 +45,8 @@
 # undef _GLIBCXX_HAVE_GETS
 #endif
 
+// Glibc 2.23 removed the obsolete isinf and isnan declarations. Check the
+// version dynamically in case it has changed since libstdc++ was configured.
+#define _GLIBCXX_NO_OBSOLETE_ISINF_ISNAN_DYNAMIC __GLIBC_PREREQ(2,23)
+
 #endif

libstdc++-v3/doc/html/manual/internals.html

@@ -88,6 +88,13 @@ the standard.
 to an expression that yields 0 if and only if the system headers
 are exposing proper support for the related set of functions.  If defined,
 it must be 0 while bootstrapping the compiler/rebuilding the library.
+   </p><p><code class="code">_GLIBCXX_NO_OBSOLETE_ISINF_ISNAN_DYNAMIC</code> may be defined
+to an expression that yields 0 if and only if the system headers
+are exposing non-standard <code class="code">isinf(double)</code> and
+<code class="code">isnan(double)</code> functions in the global namespace. Those functions
+should be detected automatically by the <code class="code">configure</code> script when
+libstdc++ is built but if their presence depends on compilation flags or
+other macros the static configuration can be overridden.
    </p><p>Finally, you should bracket the entire file in an include-guard, like
 this:
    </p><pre class="programlisting">

libstdc++-v3/doc/xml/manual/internals.xml

@@ -132,6 +132,14 @@ the standard.
 to an expression that yields 0 if and only if the system headers
 are exposing proper support for the related set of functions.  If defined,
 it must be 0 while bootstrapping the compiler/rebuilding the library.
+   </para>
+   <para><code>_GLIBCXX_NO_OBSOLETE_ISINF_ISNAN_DYNAMIC</code> may be defined
+to an expression that yields 0 if and only if the system headers
+are exposing non-standard <code>isinf(double)</code> and
+<code>isnan(double)</code> functions in the global namespace. Those functions
+should be detected automatically by the <code>configure</code> script when
+libstdc++ is built but if their presence depends on compilation flags or
+other macros the static configuration can be overridden.
    </para>
    <para>Finally, you should bracket the entire file in an include-guard, like
 this:

libstdc++-v3/include/c_global/cmath

@@ -610,7 +610,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   isinf(float __x)
   { return __builtin_isinf(__x); }
 
-#ifdef _GLIBCXX_HAVE_OBSOLETE_ISINF_ISNAN
+#if _GLIBCXX_HAVE_OBSOLETE_ISINF_ISNAN \
+  && !_GLIBCXX_NO_OBSOLETE_ISINF_ISNAN_DYNAMIC
   using ::isinf;
 #else
   constexpr bool
@@ -634,7 +635,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   isnan(float __x)
   { return __builtin_isnan(__x); }
 
-#ifdef _GLIBCXX_HAVE_OBSOLETE_ISINF_ISNAN
+#if _GLIBCXX_HAVE_OBSOLETE_ISINF_ISNAN \
+  && !_GLIBCXX_NO_OBSOLETE_ISINF_ISNAN_DYNAMIC
   using ::isnan;
 #else
   constexpr bool