-Wstrict-prototypes (C and Objective-C only)Warn if a function is declared or defined without specifying the argument types. (An old-style function definition is permitted without a warning if preceded by a declaration which specifies the argument types.)
-Wmissing-prototypes (C and Objective-C only)Warn if a global function is defined without a previous prototype declaration. This warning is issued even if the definition itself provides a prototype. The aim is to detect global functions that fail to be declared in header files.
http://gcc.gnu.org/onlinedocs/gcc-4....g-Options.html

Beim AVR Studio 6.1:
Eigenschaften->AVR/GNU C Compiler->Miscellaneous->Other flags: ....

mfg