build: If both Qt4 and Qt5 are installed, use Qt5

If both Qt4 and Qt5 development headers are installed, use Qt5. Building
against Qt5 should be encouraged as that is where active development
happens.

build-aux/m4/bitcoin_qt.m4

@@ -50,7 +50,7 @@ AC_DEFUN([BITCOIN_QT_INIT],[
   dnl enable qt support
   AC_ARG_WITH([gui],
     [AS_HELP_STRING([--with-gui@<:@=no|qt4|qt5|auto@:>@],
-    [build bitcoin-qt GUI (default=auto, qt4 tried first)])],
+    [build bitcoin-qt GUI (default=auto, qt5 tried first)])],
     [
      bitcoin_qt_want_version=$withval
      if test x$bitcoin_qt_want_version = xyes; then

configure.ac

@@ -538,7 +538,7 @@ fi
 BITCOIN_QT_INIT
 
 dnl sets $bitcoin_enable_qt, $bitcoin_enable_qt_test, $bitcoin_enable_qt_dbus
-BITCOIN_QT_CONFIGURE([$use_pkgconfig], [qt4])
+BITCOIN_QT_CONFIGURE([$use_pkgconfig], [qt5])
 
 if test x$build_bitcoin_utils$build_bitcoind$bitcoin_enable_qt$use_tests = xnononono; then
     use_boost=no

doc/build-unix.md

@@ -100,7 +100,7 @@ Dependencies for the GUI: Ubuntu & Debian
 
 If you want to build Bitcoin-Qt, make sure that the required packages for Qt development
 are installed. Either Qt 5 or Qt 4 are necessary to build the GUI.
-If both Qt 4 and Qt 5 are installed, Qt 4 will be used. Pass `--with-gui=qt5` to configure to choose Qt5.
+If both Qt 4 and Qt 5 are installed, Qt 5 will be used. Pass `--with-gui=qt4` to configure to choose Qt4.
 To build without GUI pass `--without-gui`.
 
 To build with Qt 5 (recommended) you need the following: