build: add glibc/libstdc++ back-compat stubs

glibc/libstdc++ have added new symbols in later releases. When running a new
binary against an older glibc, the run-time linker is unable to resolve the
new symbols and the binary refuses to run.

This can be fixed by adding our own versions of those functions, so that the
build-time linker does not emit undefined symbols for them.

This enables our binary releases to work on older Linux distros, while not
incurring the downsides of a fully static binary.

src/compat/glibc_compat.cpp

@@ -0,0 +1,19 @@
+#include "bitcoin-config.h"
+#include <cstddef>
+#include <sys/select.h>
+
+// Prior to GLIBC_2.14, memcpy was aliased to memmove.
+extern "C" void* memmove(void* a, const void* b, size_t c);
+extern "C" void* memcpy(void* a, const void* b, size_t c)
+{
+  return memmove(a, b, c);
+}
+
+extern "C" void __chk_fail (void) __attribute__((__noreturn__));
+extern "C" FDELT_TYPE __fdelt_warn(FDELT_TYPE a)
+{
+  if (a >= FD_SETSIZE)
+    __chk_fail ();
+  return a / __NFDBITS;
+}
+extern "C" FDELT_TYPE __fdelt_chk(FDELT_TYPE) __attribute__((weak, alias("__fdelt_warn")));

src/compat/glibcxx_compat.cpp

@@ -0,0 +1,80 @@
+#include <cstddef>
+#include <istream>
+#include <stdexcept>
+#include <typeinfo>
+
+#ifndef _GLIBCXX_USE_NOEXCEPT
+    #define _GLIBCXX_USE_NOEXCEPT throw()
+#endif
+
+namespace std {
+
+const char* bad_exception::what() const throw()
+{
+  return "std::bad_exception";
+}
+
+const char* bad_cast::what() const throw()
+{
+  return "std::bad_cast";
+}
+
+const char* bad_alloc::what() const throw()
+{
+  return "std::bad_alloc";
+}
+
+namespace __detail
+{
+struct _List_node_base
+{
+  void _M_hook(std::__detail::_List_node_base* const __position) throw () __attribute__((used))
+  {
+    _M_next = __position;
+    _M_prev = __position->_M_prev;
+    __position->_M_prev->_M_next = this;
+    __position->_M_prev = this;
+  }
+  void _M_unhook() __attribute__((used))
+  {
+    _List_node_base* const __next_node = _M_next;
+    _List_node_base* const __prev_node = _M_prev;
+    __prev_node->_M_next = __next_node;
+    __next_node->_M_prev = __prev_node;
+  }
+  _List_node_base* _M_next;
+  _List_node_base* _M_prev;
+};
+} // namespace detail
+
+template ostream& ostream::_M_insert(bool);
+template ostream& ostream::_M_insert(long);
+template ostream& ostream::_M_insert(double);
+template ostream& ostream::_M_insert(unsigned long);
+template ostream& ostream::_M_insert(const void*);
+template ostream& __ostream_insert(ostream&, const char*, streamsize);
+template istream& istream::_M_extract(long&);
+template istream& istream::_M_extract(unsigned short&);
+
+out_of_range::~out_of_range() _GLIBCXX_USE_NOEXCEPT { }
+
+// Used with permission.
+// See: https://github.com/madlib/madlib/commit/c3db418c0d34d6813608f2137fef1012ce03043d
+
+void
+ctype<char>::_M_widen_init() const {
+    char __tmp[sizeof(_M_widen)];
+    for (unsigned __i = 0; __i < sizeof(_M_widen); ++__i)
+        __tmp[__i] = __i;
+    do_widen(__tmp, __tmp + sizeof(__tmp), _M_widen);
+
+    _M_widen_ok = 1;
+    // Set _M_widen_ok to 2 if memcpy can't be used.
+    for (unsigned __i = 0; __i < sizeof(_M_widen); ++__i)
+        if (__tmp[__i] != _M_widen[__i]) {
+            _M_widen_ok = 2;
+            break;
+        }
+}
+
+}// namespace std