Suppress "unused parameter" warnings

A bunch of functions have parameters they do not use, but which cannot
be removed for API compatibility.

In syscalls_sam3.c, there are a lot of these, so this adds an "UNUSED"
macro which adds the "unused" variable attribute if supported (GCC
specific), or is just a noop on other compilers.

In CDC.cpp, there's only three of these variables, so this commit just
forces a dummy evaluation of them to suppress the warnings.

This helps towards arduino#1792.

Author: matthijskooijman

cores/arduino/USB/CDC.cpp

@@ -147,10 +147,15 @@ bool WEAK CDC_Setup(Setup& setup)
 int _serialPeek = -1;
 void Serial_::begin(uint32_t baud_count)
 {
+	// suppress "unused parameter" warning
+	(void)baud_count;
 }
 
 void Serial_::begin(uint32_t baud_count, uint8_t config)
 {
+	// suppress "unused parameter" warning
+	(void)baud_count;
+	(void)config;
 }
 
 void Serial_::end(void)

cores/arduino/syscalls_sam3.c

@@ -38,6 +38,14 @@
   #include <sys/stat.h>
 #endif
 
+// Helper macro to mark unused parameters and prevent compiler warnings.
+// Appends _UNUSED to the variable name to prevent accidentally using them.
+#ifdef __GNUC__
+#  define UNUSED(x) x ## _UNUSED __attribute__((__unused__))
+#else
+#  define UNUSED(x) x ## _UNUSED
+#endif
+
 /*----------------------------------------------------------------------------
  *        Exported variables
  *----------------------------------------------------------------------------*/
@@ -69,39 +77,39 @@ extern caddr_t _sbrk ( int incr )
     return (caddr_t) prev_heap ;
 }
 
-extern int link( char *cOld, char *cNew )
+extern int link( UNUSED(char *cOld), UNUSED(char *cNew) )
 {
     return -1 ;
 }
 
-extern int _close( int file )
+extern int _close( UNUSED(int file) )
 {
     return -1 ;
 }
 
-extern int _fstat( int file, struct stat *st )
+extern int _fstat( UNUSED(int file), struct stat *st )
 {
     st->st_mode = S_IFCHR ;
 
     return 0 ;
 }
 
-extern int _isatty( int file )
+extern int _isatty( UNUSED(int file) )
 {
     return 1 ;
 }
 
-extern int _lseek( int file, int ptr, int dir )
+extern int _lseek( UNUSED(int file), UNUSED(int ptr), UNUSED(int dir) )
 {
     return 0 ;
 }
 
-extern int _read(int file, char *ptr, int len)
+extern int _read(UNUSED(int file), UNUSED(char *ptr), UNUSED(int len) )
 {
     return 0 ;
 }
 
-extern int _write( int file, char *ptr, int len )
+extern int _write( UNUSED(int file), char *ptr, int len )
 {
     int iIndex ;
 
@@ -129,7 +137,7 @@ extern void _exit( int status )
     for ( ; ; ) ;
 }
 
-extern void _kill( int pid, int sig )
+extern void _kill( UNUSED(int pid), UNUSED(int sig) )
 {
     return ;
 }