Android variants may not have __system_property_get

Some android systems like ARM64 may not have the __system_property_get
symbol in libc (but still have it in the public headers).  Detect this
condition at build time.  The __system_property_get method of retrieving
name servers is deprecated as of Oreo so should strictly be a fallback
mechanism anyhow.

CMakeLists.txt

@@ -350,6 +350,10 @@ CHECK_SYMBOL_EXISTS (strncmpi        "${CMAKE_EXTRA_INCLUDE_FILES}" HAVE_STRNCMP
 CHECK_SYMBOL_EXISTS (strnicmp        "${CMAKE_EXTRA_INCLUDE_FILES}" HAVE_STRNICMP)
 CHECK_SYMBOL_EXISTS (writev          "${CMAKE_EXTRA_INCLUDE_FILES}" HAVE_WRITEV)
 
+# On Android, the system headers may define __system_property_get(), but excluded
+# from libc.  We need to perform a link test instead of a header/symbol test.
+CHECK_FUNCTION_EXISTS (__system_property_get HAVE___SYSTEM_PROPERTY_GET)
+
 # Unset temporary data
 SET (CMAKE_EXTRA_INCLUDE_FILES)
 SET (CMAKE_REQUIRED_DEFINITIONS)

ares_config.h.cmake

@@ -331,6 +331,9 @@
 /* Define to 1 if you have the ws2tcpip.h header file. */
 #cmakedefine HAVE_WS2TCPIP_H
 
+/* Define to 1 if you have the __system_property_get function */
+#cmakedefine HAVE___SYSTEM_PROPERTY_GET
+
 /* Define to 1 if you need the malloc.h header file even with stdlib.h */
 #cmakedefine NEED_MALLOC_H
 

ares_init.c

@@ -1641,6 +1641,7 @@ static int init_by_resolv_conf(ares_channel channel)
     ares_free(dns_servers);
   }
 
+#  ifdef HAVE___SYSTEM_PROPERTY_GET
   /* Old way using the system property still in place as
    * a fallback. Older android versions can still use this.
    * it's possible for older apps not not have added the new
@@ -1662,6 +1663,7 @@ static int init_by_resolv_conf(ares_channel channel)
       status = ARES_EOF;
     }
   }
+#  endif /* HAVE___SYSTEM_PROPERTY_GET */
 #elif defined(CARES_USE_LIBRESOLV)
   struct __res_state res;
   memset(&res, 0, sizeof(res));

configure.ac

@@ -812,6 +812,11 @@ AC_CHECK_FUNCS([bitncmp \
   ])
 ])
 
+dnl Android. Some variants like arm64 may no longer have __system_property_get
+dnl in libc, but they are defined in the headers.  Perform a link check.
+AC_CHECK_FUNC([__system_property_get], [
+    AC_DEFINE([HAVE___SYSTEM_PROPERTY_GET], [1], [Define if __system_property_get exists.])
+])
 
 dnl Check if the getnameinfo function is available
 dnl and get the types of five of its arguments.