Merge bitcoin#17396: build: modest Android improvements

366913e build: AX_BOOST_THREAD serial 33 (Igor Cota)
cf06811 build: disable D-Bus on Android by default (Igor Cota)

Pull request description:

  I've been trying to build for Android on different OSes/Gitian with varying success. Build system is quite the beast and sometimes it doesn't get it right. To make sure it does these three little tweaks make the Android build more robust:

  - disable D-Bus (Android doesn't support it and has its own way to trigger notifications)
  - don't flag `-lpthread` when linking Boost, [Bionic has built-in support](https://stackoverflow.com/questions/30801752/android-ndk-and-pthread)
  - ~~add `-static-libstdc++` to linker flags. This avoids having to bundle `libc++_shared` with CLI apps, still necessary with `bitcoin-qt` though (thanks Sjors)~~

  I think these are small and fairly straightforward so I put them all into this one PR.

ACKs for top commit:
  fanquake:
    ACK 366913e

Tree-SHA512: 31465fd228a5877c20aa2a05f98242d4eeb328b9b35bd1a7a3dcfb1ef51379d84053a81ade5a65436ffc1bc8ccd21f11ed0539eb10e827d182c0c04394629af0

build-aux/m4/ax_boost_thread.m4

@@ -30,7 +30,7 @@
 #   and this notice are preserved. This file is offered as-is, without any
 #   warranty.
 
-#serial 32
+#serial 33
 
 AC_DEFUN([AX_BOOST_THREAD],
 [
@@ -67,13 +67,24 @@ AC_DEFUN([AX_BOOST_THREAD],
         [AC_LANG_PUSH([C++])
              CXXFLAGS_SAVE=$CXXFLAGS
 
-             if test "x$host_os" = "xsolaris" ; then
-                 CXXFLAGS="-pthreads $CXXFLAGS"
-             elif test "x$host_os" = "xmingw32" ; then
-                 CXXFLAGS="-mthreads $CXXFLAGS"
-             else
-                CXXFLAGS="-pthread $CXXFLAGS"
-             fi
+             case "x$host_os" in
+                 xsolaris )
+                     CXXFLAGS="-pthreads $CXXFLAGS"
+                     break;
+                     ;;
+                 xmingw32 )
+                     CXXFLAGS="-mthreads $CXXFLAGS"
+                     break;
+                     ;;
+                 *android* )
+                     break;
+                     ;;
+                 * )
+                     CXXFLAGS="-pthread $CXXFLAGS"
+                     break;
+                     ;;
+             esac
+
              AC_COMPILE_IFELSE([
                  AC_LANG_PROGRAM(
                      [[@%:@include <boost/thread/thread.hpp>]],
@@ -84,13 +95,23 @@ AC_DEFUN([AX_BOOST_THREAD],
              AC_LANG_POP([C++])
         ])
         if test "x$ax_cv_boost_thread" = "xyes"; then
-           if test "x$host_os" = "xsolaris" ; then
-              BOOST_CPPFLAGS="-pthreads $BOOST_CPPFLAGS"
-           elif test "x$host_os" = "xmingw32" ; then
-              BOOST_CPPFLAGS="-mthreads $BOOST_CPPFLAGS"
-           else
-              BOOST_CPPFLAGS="-pthread $BOOST_CPPFLAGS"
-           fi
+            case "x$host_os" in
+                xsolaris )
+                    BOOST_CPPFLAGS="-pthreads $BOOST_CPPFLAGS"
+                    break;
+                    ;;
+                xmingw32 )
+                    BOOST_CPPFLAGS="-mthreads $BOOST_CPPFLAGS"
+                    break;
+                    ;;
+                *android* )
+                    break;
+                    ;;
+                * )
+                    BOOST_CPPFLAGS="-pthread $BOOST_CPPFLAGS"
+                    break;
+                    ;;
+            esac
 
             AC_SUBST(BOOST_CPPFLAGS)
 
@@ -148,6 +169,9 @@ AC_DEFUN([AX_BOOST_THREAD],
                     xmingw32 )
                         break;
                         ;;
+                    *android* )
+                        break;
+                        ;;
                     * )
                         BOOST_THREAD_LIB="$BOOST_THREAD_LIB -lpthread"
                         break;

build-aux/m4/bitcoin_qt.m4

@@ -79,10 +79,19 @@ AC_DEFUN([BITCOIN_QT_INIT],[
 
   AC_ARG_WITH([qtdbus],
     [AS_HELP_STRING([--with-qtdbus],
-    [enable DBus support (default is yes if qt is enabled and QtDBus is found)])],
+    [enable DBus support (default is yes if qt is enabled and QtDBus is found, except on Android)])],
     [use_dbus=$withval],
     [use_dbus=auto])
 
+  dnl Android doesn't support D-Bus and certainly doesn't use it for notifications
+  case $host in
+    *android*)
+      if test "x$use_dbus" != xyes; then
+        use_dbus=no
+      fi
+    ;;
+  esac
+
   AC_SUBST(QT_TRANSLATION_DIR,$qt_translation_path)
 ])