fpclassify() is C99, not necessarily available in C++

(cherry picked from commit d23fec0d)

modules/audio_filter/spatializer/Modules.am

 SOURCES_spatializer = spatializer.cpp \
     allpass.cpp allpass.hpp comb.cpp comb.hpp \
-    denormals.h tuning.h revmodel.cpp revmodel.hpp
+    denormals.h denormals.c tuning.h revmodel.cpp revmodel.hpp
 

modules/audio_filter/spatializer/denormals.h

@@ -8,16 +8,11 @@
 #ifndef _denormals_
 #define _denormals_
 
-#include <stdint.h>
 
-#include <math.h>
-
-static inline float undenormalise( float f )
-{
-    if( fpclassify( f ) == FP_SUBNORMAL  )
-        return 0.0;
-    return f;
-}
+#ifdef __cplusplus
+extern "C"
+#endif
+float undenormalise( float );
 
 #endif//_denormals_