Fixes to makefiles

- Add a warning when using the systems taglib less or equal to version 1.11.1
- Remove OpenGL and Qt5::OpenGL dependency, it is not used by the analyzer
- Rename some variables

src/analyzer/analyzer.cpp

@@ -1,20 +0,0 @@
-#include "config.h"
-
-#include <QWidget>
-#include <QGLWidget>
-#include <QVector>
-
-#include "analyzer.h"
-
-#include "engine/enginebase.h"
-
-AnalyzerBase::AnalyzerBase(QWidget *parent)
-    : QGLWidget(parent), engine_(nullptr) {}
-
-void AnalyzerBase::set_engine(Engine::Base *engine) {
-  disconnect(engine_);
-  engine_ = engine;
-  if (engine_) {
-    connect(engine_, SIGNAL(SpectrumAvailable(const QVector<float>&)), SLOT(SpectrumAvailable(const QVector<float>&)));
-  }
-}