new R check warnings for nma() functions

[Barry Grant]

nma: no visible global function definition for ‘cmpfun’ nma: multiple local function definitions for ‘ff’ with different formal arguments

[Lars Skjærven]

Is it OK to add package 'compiler'? I use it to byte compile code for building the hessian matrix. If so, then this issue is resolved.

'compiler' Is quite neat and it provides a nice speedup in many cases. it is included from R versions >= 2.13.

[Xinqiu Yao]

I have run R check again and the warnings disappear - Xinqiu

[Barry Grant]

I think this is fine also - thanks Lars!