Clean up math functions

- Remove checking for standard functions from the build system
- Prefix all invocations with std::
- Replace suffixed functions with unprefixed versions
- Include <cmath> in all files that use it and remove the global include

e.g. floorf(x) -> std::floor(x)

src/RageSoundUtil.cpp

@@ -2,6 +2,8 @@
 #include "RageSoundUtil.h"
 #include "RageUtil.h"
 
+#include <cmath>
+
 void RageSoundUtil::Attenuate( float *pBuf, int iSamples, float fVolume )
 {
 	while( iSamples-- )
@@ -91,7 +93,7 @@ void RageSoundUtil::ConvertFloatToNativeInt16( const float *pFrom, int16_t *pTo,
 {
 	for( int i = 0; i < iSamples; ++i )
 	{
-		int iOut = lrintf( pFrom[i] * 32768.0f );
+		int iOut = std::lrint( pFrom[i] * 32768.0f );
 		pTo[i] = clamp( iOut, -32768, 32767 );
 	}
 }