Merge #13458: gui: Drop qt4 support

af6ac3b677454644364fd24d0df0c02ac9b8c8db doc: Remove mention of Qt4 from build docs (Wladimir J. van der Laan)
462c71f71b5d753bc8327ab833dea23679450ca3 test: Update travis to not test Qt4 anymore (Wladimir J. van der Laan)
907f73bbc5b6c98b01d7c6088a294dea66634a3f gui: Remove QT_VERSION fallbacks for Qt < 5 (Wladimir J. van der Laan)
bad068ad9f4bc60bfc10e27d4ffaec92d7df8491 build: Build system changes to support only Qt5 (Wladimir J. van der Laan)

Pull request description:

  Implements #8263.

  Qt4.x has been EOL since 2015, and at least Gentoo has, or is going to drop support for it. I wouldn't be surprised if other Linux distributions follow.

  This removes Qt4 detection from the build system, as well as removes all Qt4 fallbacks from the code. Turns out there's more than I expected: this is going to make maintenance of the GUI code, as well as adding new features significantly easier.

  (I know there's still some references left to qt4 in RPM and Debian build script, but I don't have the knowledge how to fix them)

Tree-SHA512: d495924fd4dda6f6566ba44ee96be7cbe62e69ba1ca993b80a8449f78da852b7f1bd3e8200d57cfa1d72233c340eeff4596fb0032ecbddc715d99aea63817d3f

3 files changed, 4 insertions, 11 deletions

diff --git a/doc/build-osx.md b/doc/build-osx.md
index abd305cf9a..a07dbd1e1d 100644
--- a/doc/build-osx.md
+++ b/doc/build-osx.md
@@ -24,8 +24,6 @@ If you want to build the disk image with `make deploy` (.dmg / optional), you ne
 
     brew install librsvg
 
-NOTE: Building with Qt4 is still supported, however, could result in a broken UI. Building with Qt5 is recommended.
-
 Berkeley DB
 -----------
 It is recommended to use Berkeley DB 4.8. If you have to build it yourself,
diff --git a/doc/build-unix.md b/doc/build-unix.md
index 1ab5659511..e884c0ab67 100644
--- a/doc/build-unix.md
+++ b/doc/build-unix.md
@@ -100,18 +100,13 @@ ZMQ dependencies (provides ZMQ API 4.x):
 #### Dependencies for the GUI
 
 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 5 will be used. Pass `--with-gui=qt4` to configure to choose Qt4.
+are installed. Qt 5 is necessary to build the GUI.
 To build without GUI pass `--without-gui`.
 
-To build with Qt 5 (recommended) you need the following:
+To build with Qt 5 you need the following:
 
     sudo apt-get install libqt5gui5 libqt5core5a libqt5dbus5 qttools5-dev qttools5-dev-tools libprotobuf-dev protobuf-compiler
 
-Alternatively, to build with Qt 4 you need the following:
-
-    sudo apt-get install libqt4-dev libprotobuf-dev protobuf-compiler
-
 libqrencode (optional) can be installed with:
 
     sudo apt-get install libqrencode-dev
@@ -132,7 +127,7 @@ Optional:
 
     sudo dnf install miniupnpc-devel
 
-To build with Qt 5 (recommended) you need the following:
+To build with Qt 5 you need the following:
 
     sudo dnf install qt5-qttools-devel qt5-qtbase-devel protobuf-devel
 
diff --git a/doc/dependencies.md b/doc/dependencies.md
index 7aa96c4c9b..793c659419 100644
--- a/doc/dependencies.md
+++ b/doc/dependencies.md
@@ -23,7 +23,7 @@ These are the dependencies currently used by Bitcoin Core. You can find instruct
 | protobuf | [2.6.3](https://github.com/google/protobuf/releases) |  | No |  |  |
 | Python (tests) |  | [3.4](https://www.python.org/downloads) |  |  |  |
 | qrencode | [3.4.4](https://fukuchi.org/works/qrencode) |  | No |  |  |
-| Qt | [5.7.1](https://download.qt.io/official_releases/qt/) | 4.7+ | No |  |  |
+| Qt | [5.7.1](https://download.qt.io/official_releases/qt/) | 5.x | No |  |  |
 | XCB |  |  |  |  | [Yes](https://github.com/bitcoin/bitcoin/blob/master/depends/packages/qt.mk#L94) (Linux only) |
 | xkbcommon |  |  |  |  | [Yes](https://github.com/bitcoin/bitcoin/blob/master/depends/packages/qt.mk#L93) (Linux only) |
 | ZeroMQ | [4.2.3](https://github.com/zeromq/libzmq/releases) |  | No |  |  |