Link with -ldl option only when it is supported

-ldl option was used unconditionally while libdl is not supported
when libc is static.
Add build test to configure which checks if libdl is supported.
QMAKE_LIBS_DYNLOAD in "src/corelib/plugin/plugin.pri" is now used only
if libdl is available.
qt_linux_find_symbol_sys from qlibrary_unix is now used only if
QT_NO_DYNAMIC_LIBRARY is not defined.

Initially reported by Buildroot autobuilder here:
http://autobuild.buildroot.net/results/a85/a85a1839a45fb6102e53131ecc8f6dadf92bcdc2

Change-Id: I0397472456efdc4f3ab5f24d01253bee8048a9d1
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

config.tests/unix/libdl/libdl.cpp

@@ -0,0 +1,39 @@
+/****************************************************************************
+**
+** Copyright (C) 2015 The Qt Company Ltd.
+** Contact: http://www.qt.io/licensing/
+**
+** This file is part of the config.tests 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 <dlfcn.h>
+
+int main(int, char **)
+{
+    dlopen(0, 0);
+}

config.tests/unix/libdl/libdl.pro

@@ -0,0 +1,3 @@
+SOURCES = libdl.cpp
+CONFIG -= qt dylib
+LIBS += -ldl

configure

@@ -4734,6 +4734,12 @@ if [ "$CFG_LIBPNG" = "auto" ]; then
     fi
 fi
 
+# detect dl
+if ! compileTest unix/libdl "libdl"; then
+    QMakeVar add DEFINES QT_NO_DYNAMIC_LIBRARY
+    QMAKE_CONFIG="$QMAKE_CONFIG no-libdl"
+fi
+
 if [ "$CFG_EGLFS" = "yes" ]; then
     if [ "$CFG_EGL" = "no" ]; then
         echo "The EGLFS plugin requires EGL support and cannot be built"

src/corelib/plugin/plugin.pri

@@ -35,4 +35,4 @@ integrity {
 	SOURCES += plugin/qlibrary_unix.cpp
 }
 
-LIBS_PRIVATE += $$QMAKE_LIBS_DYNLOAD
+!no-libdl: LIBS_PRIVATE += $$QMAKE_LIBS_DYNLOAD

src/corelib/plugin/qlibrary_unix.cpp

@@ -286,7 +286,7 @@ bool QLibraryPrivate::unload_sys()
     return true;
 }
 
-#ifdef Q_OS_LINUX
+#if defined(Q_OS_LINUX) && !defined(QT_NO_DYNAMIC_LIBRARY)
 Q_CORE_EXPORT QFunctionPointer qt_linux_find_symbol_sys(const char *symbol)
 {
     return QFunctionPointer(dlsym(RTLD_DEFAULT, symbol));