build: avoid getifaddrs when unavailable

Summary:
```
These changes from @theuni help building when targeting platforms that
don't always have getifaddrs available like Android < 24
```

Backport of core PR14127
https://github.com/bitcoin/bitcoin/pull/14127/files

Test Plan:
  make check
  ninja check

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3824

configure.ac

@@ -704,6 +704,10 @@ fi
 
 AC_CHECK_HEADERS([endian.h sys/endian.h byteswap.h stdio.h stdlib.h unistd.h strings.h sys/types.h sys/stat.h sys/select.h sys/prctl.h])
 
+AC_CHECK_DECLS([getifaddrs, freeifaddrs],,,
+    [#include <sys/types.h>
+    #include <ifaddrs.h>]
+)
 AC_CHECK_DECLS([strnlen])
 
 # Check for daemon(3), unrelated to --with-daemon (although used by it)

src/config/CMakeLists.txt

@@ -127,6 +127,10 @@ check_symbol_exists(daemon "unistd.h" HAVE_DECL_DAEMON)
 check_symbol_exists(getentropy "unistd.h" HAVE_GETENTROPY)
 check_symbol_exists(getentropy "sys/random.h" HAVE_GETENTROPY_RAND)
 
+# getifaddrs and freeifaddrs may be unavailable with some Android versions
+check_symbol_exists(getifaddrs "sys/types.h;ifaddrs.h" HAVE_DECL_GETIFADDRS)
+check_symbol_exists(freeifaddrs "sys/types.h;ifaddrs.h" HAVE_DECL_FREEIFADDRS)
+
 check_cxx_source_compiles("
 	#include <unistd.h>  /* for syscall */
 	#include <sys/syscall.h>  /* for SYS_getrandom */

src/config/bitcoin-config.h.cmake.in

@@ -45,6 +45,8 @@
 
 #cmakedefine HAVE_DECL_STRNLEN 1
 #cmakedefine HAVE_DECL_DAEMON 1
+#cmakedefine HAVE_DECL_GETIFADDRS 1
+#cmakedefine HAVE_DECL_FREEIFADDRS 1
 #cmakedefine HAVE_GETENTROPY 1
 #cmakedefine HAVE_GETENTROPY_RAND 1
 #cmakedefine HAVE_SYS_GETRANDOM 1

src/net.cpp

@@ -2301,7 +2301,7 @@ void Discover() {
             }
         }
     }
-#else
+#elif (HAVE_DECL_GETIFADDRS && HAVE_DECL_FREEIFADDRS)
     // Get local host ip
     struct ifaddrs *myaddrs;
     if (getifaddrs(&myaddrs) == 0) {