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/AutoKeysounds.cpp

@@ -322,8 +322,8 @@ void AutoKeysounds::Update( float fDelta )
 		iRowNow = std::max( 0, iRowNow );
 		static int iRowLastCrossed = 0;
 
-		float fBeatLast = roundf(NoteRowToBeat(iRowLastCrossed));
-		float fBeatNow = roundf(NoteRowToBeat(iRowNow));
+		float fBeatLast = std::round(NoteRowToBeat(iRowLastCrossed));
+		float fBeatNow = std::round(NoteRowToBeat(iRowNow));
 
 		bCrossedABeat = fBeatLast != fBeatNow;