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/RageSoundMixBuffer.cpp

@@ -2,6 +2,8 @@
 #include "RageSoundMixBuffer.h"
 #include "RageUtil.h"
 
+#include <cmath>
+
 #if defined(MACOSX)
 #include "archutils/Darwin/VectorHelper.h"
 #ifdef USE_VEC
@@ -79,7 +81,7 @@ void RageSoundMixBuffer::read( int16_t *pBuf )
 	{
 		float iOut = m_pMixbuf[iPos];
 		iOut = clamp( iOut, -1.0f, +1.0f );
-		pBuf[iPos] = lrintf(iOut * 32767);
+		pBuf[iPos] = std::lrint(iOut * 32767);
 	}
 	m_iBufUsed = 0;
 }