Some C compilers produce warnings or compilation errors if an attempt

Some C compilers produce warnings or compilation errors if an attempt
is made to directly cast a function of one type to what it considers and
incompatible type. In particular gcc 3.4.2.

Add new openssl_fcast macro to place functions into a form where the compiler
will allow them to be cast.

The current version achives this by casting to: void function(void).