Add functions randomSeed and random to WMath.cpp

These functions use stdlib.h to generate random numbers
Some refactoring in WMath.cpp
Change location of include in virtual_main

The header WMath.h belongs inside the condition #ifdef __cplusplus
since the source is a .cpp file.
Shift declarations of delay functions to wiring.h

The definitions of the delay functions were already shifted to wiring.c
but the prototypes were still present in sysfs.h