projectM: Force building with a specific older C++ version

This fixes errors like these:

libprojectM/projectM.hpp:303:8: error: no template named 'auto_ptr' in namespace 'std'
  std::auto_ptr<Preset> m_activePreset;
  ~~~~~^

The std::auto_ptr type was deprecated in C++11 and removed in C++17.

The VLC C++ headers use C++11 features, so the only working
alternatives here are C++11 or C++14.

modules/visualization/Makefile.am

@@ -19,7 +19,7 @@ EXTRA_LTLIBRARIES += libgoom_plugin.la
 visu_LTLIBRARIES += $(LTLIBgoom)
 
 libprojectm_plugin_la_SOURCES = visualization/projectm.cpp
-libprojectm_plugin_la_CXXFLAGS = $(AM_CXXFLAGS) $(PROJECTM_CFLAGS)
+libprojectm_plugin_la_CXXFLAGS = $(AM_CXXFLAGS) $(PROJECTM_CFLAGS) -std=gnu++14
 libprojectm_plugin_la_LDFLAGS = $(AM_LDFLAGS) -rpath '$(visudir)'
 libprojectm_plugin_la_LIBADD = $(PROJECTM_LIBS)
 EXTRA_LTLIBRARIES += libprojectm_plugin.la