Try to load udev at runtime if we do not link against it

Initial patch attached [1], but since a non-udev build with Qt 4 crashes, it causes a regression for those cases, so that crash has to be fixed first.

Once that is done, this patch can be applied.

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7ddc138 in udev_list_entry_get_name () from /usr/lib/libQtSerialPort.so.1
(gdb) bt
#0 0x00007ffff7ddc138 in udev_list_entry_get_name () from /usr/lib/libQtSerialPort.so.1
#1 0x00007ffff392f4c0 in ?? () from /usr/lib/libudev.so.1
#2 0x00007ffff392fc89 in ?? () from /usr/lib/libudev.so.1
#3 0x00007ffff39304f0 in udev_enumerate_scan_devices () from /usr/lib/libudev.so.1
#4 0x00007ffff7bcec74 in availablePortsByUdev () at qserialportinfo_unix.cpp:229
#5 0x00007ffff7bcf55d in QSerialPortInfo::availablePorts () at qserialportinfo_unix.cpp:323
#6 0x0000000000401f3e in main (argc=1, argv=0x7fffffffe798) at main.cpp:52
(gdb)

I compared the symbols with Qt 4 and Qt 5 and they have the same offset values. It is getting peculiar...

I also tried the following change to explicitly resemble the Qt 5 operation, but did not help:

diff --git a/src/serialport/qtudev_p.h b/src/serialport/qtudev_p.h
index 1187165..229efb5 100644
--- a/src/serialport/qtudev_p.h
+++ b/src/serialport/qtudev_p.h
@@ -96,10 +96,13 @@ inline QFunctionPointer resolveSymbol(const char *symbolName)
 {
     QFunctionPointer symbolFunctionPointer = udevLibrary.resolve(symbolName);
 #else
-inline void *resolveSymbol(const char *symbolName)
+typedef void (*QFunctionPointer)();
+inline QFunctionPointer resolveSymbol(const char *symbolName)
 {
-    void *symbolFunctionPointer = udevLibrary.resolve(symbolName);
+    QFunctionPointer symbolFunctionPointer = QFunctionPointer(udevLibrary.resolve(symbolName));
 #endif
+    printf("Symbol name: %s\n", symbolName);
+    printf("Symbol address: %p\n", symbolFunctionPointer);
     if (!symbolFunctionPointer)
         qWarning("Failed to resolve the udev symbol: %s", symbolName);

[1] https://codereview.qt-project.org/#change,72143