ircd::allocator: Support mallinfo2() for deprecated mallinfo(3) for GNU allocator.

configure.ac

@@ -1272,6 +1272,8 @@ AC_CHECK_FUNCS([          \
 	backtrace             \
 	fadvise               \
 	madvise               \
+	mallinfo              \
+	mallinfo2             \
 	mlock                 \
 	mlock2                \
 	mmap                  \

ircd/allocator_gnu.cc

@@ -47,7 +47,13 @@ ircd::allocator::info(const mutable_buffer &buf,
 
 	const auto ma
 	{
-		::mallinfo()
+		#if defined(HAVE_MALLINFO2)
+			::mallinfo2()
+		#elif defined(HAVE_MALLINFO)
+			::mallinfo2()
+		#else
+			#error "Missing mallinfo(3)."
+		#endif
 	};
 
 	char pbuf[96];