The prototype for strcasecmp() is apparently in strings.h.

src/include/strings.h

 #define fls( x ) \
 	( __builtin_constant_p ( x ) ? __constant_fls ( x ) : __fls ( x ) )
 
+extern int strcasecmp ( const char *s1, const char *s2 );
+
 #endif /* _STRINGS_H */