Add a qt_version_tag symbol to QtCore that uses ELF versions

This symbol will exist with different ELF versions, allowing us to know
which version of Qt a given library or application was linked
against. When this symbol gets used, automatic packaging tools will
extract the dependency. The trick here is that the application uses
qt_version_tag without knowing which version is current, but the linker
resolves it to the current version and records that.

For example, if this were used with Qt 5.5, RPM's find-requires on an
application using QtCore would print:

  libQt5Core.so.5()(64bit)
  libQt5Core.so.5(Qt_5.5)(64bit)
  libQt5Core.so.5(Qt_5)(64bit)

Whereas find-provides on QtCore would print:

  libQt5Core.so.5()(64bit)
  libQt5Core.so.5(libQt5Core.so.5)(64bit)
  libQt5Core.so.5(Qt_5.0)(64bit)
  libQt5Core.so.5(Qt_5.1)(64bit)
  libQt5Core.so.5(Qt_5.2)(64bit)
  libQt5Core.so.5(Qt_5.3)(64bit)
  libQt5Core.so.5(Qt_5.4)(64bit)
  libQt5Core.so.5(Qt_5.5)(64bit)
  libQt5Core.so.5(Qt_5)(64bit)

Therefore, automatic dependency resolution would have the information it
needs to conclude that the application requires Qt >= 5.5.

Change-Id: I049a653beeb5454c9539ffff13e3fec9aeb50197
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/global.pri

@@ -27,7 +27,8 @@ SOURCES += \
 	global/qmalloc.cpp \
         global/qnumeric.cpp \
         global/qlogging.cpp \
-        global/qhooks.cpp
+        global/qhooks.cpp \
+        global/qversiontagging.cpp
 
 # qlibraryinfo.cpp includes qconfig.cpp
 INCLUDEPATH += $$QT_BUILD_TREE/src/corelib/global

src/corelib/global/qversiontagging.cpp

@@ -0,0 +1,62 @@
+/****************************************************************************
+**
+** Copyright (C) 2015 Intel Corporation.
+** Contact: http://www.qt.io/licensing/
+**
+** This file is part of the QtCore module of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL21$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see http://www.qt.io/terms-conditions. For further
+** information use the contact form at http://www.qt.io/contact-us.
+**
+** GNU Lesser General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU Lesser
+** General Public License version 2.1 or version 3 as published by the Free
+** Software Foundation and appearing in the file LICENSE.LGPLv21 and
+** LICENSE.LGPLv3 included in the packaging of this file. Please review the
+** following information to ensure the GNU Lesser General Public License
+** requirements will be met: https://www.gnu.org/licenses/lgpl.html and
+** http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
+**
+** As a special exception, The Qt Company gives you certain additional
+** rights. These rights are described in The Qt Company LGPL Exception
+** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#include "qglobal.h"
+
+#if defined(Q_CC_GNU) && defined(Q_OS_LINUX) && defined(Q_PROCESSOR_X86) && !defined(QT_STATIC)
+# define SYM QT_MANGLE_NAMESPACE(qt_version_tag)
+# define SSYM QT_STRINGIFY(SYM)
+
+asm(
+// ASM macro that makes one ELF versioned symbol qt_version_tag{sep}Qt_{major}.{minor}
+// that is an alias to qt_version_tag_{major}_{minor}.
+// The {sep} parameter must be @ for all old versions and @@ for the current version.
+".macro      make_one_tag    major minor sep\n"
+".globl      " SSYM "_\\major\\()_\\minor\n"                        // make the symbol global
+SSYM "_\\major\\()_\\minor:\n"                                      // declare it
+"    .symver " SSYM "_\\major\\()_\\minor, " SSYM "\\sep\\()Qt_\\major\\().\\minor\n"
+".endm\n"
+
+".altmacro\n"
+".bss\n"
+".set qt_version_major, " QT_STRINGIFY(QT_VERSION) " >> 16\n"       // set qt_version_major
+".set qt_version_minor, 0\n"                                        // set qt_version_minor to 0 (it will grow to the current)
+".rept (" QT_STRINGIFY(QT_VERSION) " >> 8) & 0xff\n"                // repeat minor version times (0 to N-1)
+"    make_one_tag    %qt_version_major, %qt_version_minor, @\n"
+"    .set qt_version_minor, (qt_version_minor + 1)\n"
+".endr\n"
+"    make_one_tag    %qt_version_major, %qt_version_minor, @@\n"    // call the macro for the current version
+"    .space  1\n"                                                   // variable is 1 byte, value 0
+);
+
+#endif