LV2: fixed compilation without LV2 support or older versions of LV2

LV2 core was not compiled conditionally into LMMS binary so compilation
failed when building without LV2 support. Furthermore added
compatibility code for SLV2 <= 0.6.2.

include/lv2_2_lmms.h

@@ -29,6 +29,7 @@
 
 #include "lv2_manager.h"
 
+#ifdef LMMS_HAVE_LV2
 
 class EXPORT lv22LMMS : public lv2Manager
 {
@@ -76,3 +77,5 @@ private:
 } ;
 
 #endif
+
+#endif

include/lv2_manager.h

@@ -26,6 +26,8 @@
 #ifndef _LV2_MANAGER_H
 #define _LV2_MANAGER_H
 
+#ifdef LMMS_HAVE_LV2
+
 #include <slv2/world.h>
 #include <slv2/plugin.h>
 #include <slv2/scalepoint.h>
@@ -378,3 +380,5 @@ private:
 extern lv2Manager * static_lv2_manager; // There is only one of these...
 
 #endif
+
+#endif