Remove logic trying to guess whether it should be lib or lib64.

Instead use qmake IDE_LIBRARY_BASENAME=lib64 to set it to lib64.

qtcreator.pri

@@ -35,12 +35,12 @@ isEmpty(TEST):CONFIG(debug, debug|release) {
     }
 }
 
-linux-*-64 {
-    IDE_LIBRARY_BASENAME = lib64
-} else {
+isEmpty(IDE_LIBRARY_BASENAME) {
     IDE_LIBRARY_BASENAME = lib
 }
 
+DEFINES += IDE_LIBRARY_BASENAME=\\\"$$IDE_LIBRARY_BASENAME\\\"
+
 equals(TEST, 1) {
     QT +=testlib
     DEFINES += WITH_TESTS

src/app/main.cpp

@@ -182,11 +182,7 @@ static inline QStringList getPluginPaths()
     // 1) "plugins" (Win/Linux)
     QString pluginPath = rootDirPath;
     pluginPath += QDir::separator();
-#if defined(QT_ARCH_X86_64) && defined(Q_OS_UNIX) && !defined(Q_OS_MAC)
-    pluginPath += QLatin1String("lib64");
-#else
-    pluginPath += QLatin1String("lib");
-#endif
+    pluginPath += QLatin1String(IDE_LIBRARY_BASENAME);
     pluginPath += QDir::separator();
     pluginPath += QLatin1String("qtcreator");
     pluginPath += QDir::separator();