Switch to stdbool.h

- Add `#include <stdbool.h> to all files previously using the
   `TRUE`/`FALSE` macros
 - Replace all instances of `TRUE`/`FALSE` with `true`/`false`,
   respectively
 - Definitions of `TRUE`/`FALSE` macros retained in wiringPi.h for compatibility.

wiringPi/mcp23017.c

@@ -23,6 +23,7 @@
  */
 
 #include <stdio.h>
+#include <stdbool.h>
 #include <pthread.h>
 
 #include "wiringPi.h"
@@ -177,7 +178,7 @@ int mcp23017Setup (const int pinBase, const int i2cAddress)
   struct wiringPiNodeStruct *node ;
 
   if ((fd = wiringPiI2CSetup (i2cAddress)) < 0)
-    return FALSE ;
+    return false ;
 
   wiringPiI2CWriteReg8 (fd, MCP23x17_IOCON, IOCON_INIT) ;
 
@@ -191,5 +192,5 @@ int mcp23017Setup (const int pinBase, const int i2cAddress)
   node->data2           = wiringPiI2CReadReg8 (fd, MCP23x17_OLATA) ;
   node->data3           = wiringPiI2CReadReg8 (fd, MCP23x17_OLATB) ;
 
-  return TRUE ;
+  return true ;
 }