QGIS 3 is only qt5 compliant

Remove qt4 mention (for linux based processes)

INSTALL

@@ -1,26 +1,24 @@
 QGIS
 Building QGIS from source - step by step
-Monday January 09, 2017
+Wednesday January 25, 2017
 
 
-Last Updated: Monday January 09, 2017
-Last Change : Monday January 09, 2017
+Last Updated: Wednesday January 25, 2017
+Last Change : Friday January 20, 2017
 
 
   1. Introduction
   2. Overview
   3. Building on GNU/Linux
-    3.1. Building QGIS with Qt 4.x
+    3.1. Building QGIS with Qt 5.x
     3.2. Prepare apt
     3.3. Install build dependencies
     3.4. Setup ccache (Optional)
     3.5. Prepare your development environment
     3.6. Check out the QGIS Source Code
     3.7. Starting the compile
     3.8. Building Debian packages
-    3.9. Building QGIS with Qt 5.x
-    3.10. On Debian Linux
-    3.11. On Fedora Linux
+    3.9. On Fedora Linux
   4. Building on Windows
     4.1. Building with Microsoft Visual Studio
     4.2. Building using MinGW
@@ -139,13 +137,13 @@ those formats in GDAL.
   ========================
 
 
-  3.1. Building QGIS with Qt 4.x
+  3.1. Building QGIS with Qt 5.x
   ==============================
 
 Requires: Ubuntu / Debian derived distro
 
 /!\ Note: Refer to the section Building Debian packages for building
-debian packages.  Unless you plan to develop on QGIS, that is probably the
+debian packages. Unless you plan to develop on QGIS, that is probably the
 easiest option to compile and install QGIS.
 
 These notes are for Ubuntu - other versions and Debian derived distros may
@@ -263,6 +261,11 @@ be pressed several times before the 'g' option becomes available.
 After the 'g' generation is complete, press 'q' to exit the ccmake
 interactive dialog.
 
+/!\ Warning: Make sure that your build directory is completely empty when you 
+enter the command. Do never try to "re-use" an existing Qt4 build directory.
+If you want to use `ccmake` or other interactive tools, run the command in
+the empty build directory once before starting to use the interactive tools.
+
 Now on with the build:
 
   make -jX
@@ -349,56 +352,15 @@ Install them using dpkg.  E.g.:
   sudo debi
 
 
-  3.9. Building QGIS with Qt 5.x
-  ==============================
-
-/!\ Warning: Qt 5 is not officially supported. Building against Qt 5
-libraries seems to work quite well already but be prepared that you are one
-of very few with this setup. We strongly recommend using QGIS with Qt 4 for
-production work.
-
-Python bindings in particular are not expected to be compatible. When Qt 5
-is enabled, the build system automatically activates Python 3 and PyQt5 as well.
-There are no plugins and not even the python code shipped along the QGIS source
-code is compatible with PyQt5.
-
-You have been warned.
-
-
-  3.10. On Debian Linux
-  =====================
-
-We assume that you have the source code of QGIS ready and created a new
-subdirectory called `build` or `build-qt5` in it.
-
-
-    3.10.1. Install build dependencies
-    ==================================
-
-  apt-get install git build-essential cmake flex bison pyqt5-dev qttools5-dev qtpositioning5-dev libqt5svg5-dev libqt5webkit5-dev  libqt5gui5 libqt5scripttools5 qtscript5-dev libqca-qt5-2-dev grass-dev libgeos-dev libgdal-dev libqt5xmlpatterns5-dev libqt5scintilla2-dev pyqt5.qsci-dev python3-pyqt5.qsci libgsl-dev txt2tags libproj-dev libqwt-qt5-dev libspatialindex-dev pyqt5-dev-tools qttools5-dev-tools qt5-default python3-future python3-pyqt5.qtsql python3-psycopg2 python3-sip-dev
-
-Make sure that your build directory is completely empty when you enter the
-following command. Do never try to "re-use" an existing Qt4 build directory.
-If you want to use `ccmake` or other interactive tools, run the following
-command in the empty build directory once before starting to use the
-interactive tools.
-
-  cmake
-
-If everything went ok you can finally start to compile. (As usual append a -jX where X is the number of available cores option to make to speed up your build process)
-
-  make
-
-
-  3.11. On Fedora Linux
-  =====================
+  3.9. On Fedora Linux
+  ====================
 
 We assume that you have the source code of QGIS ready and created a
 new subdirectory called `build` or `build-qt5` in it.
 
 
-    3.11.1. Install build dependencies
-    ==================================
+    3.9.1. Install build dependencies
+    =================================
 
   dnf install qt5-qtwebkit-devel qt5-qtlocation-devel qt5-qttools-static
   qt5-qtscript-devel qca-qt5-devel python3-qt5-devel python3-qscintilla-qt5-devel