Introduce constexpr for the XBeeAddress64 constructors

This allows the compiler to better optimize (global) XBeeAdress64
instances.

With the examples sketches, this saves between 44 and 114 bytes of
program space, compiling for the Uno. Even sketches that do not directly
use any addresses shrink, because the initialization of
RemoteAtCommandRequest::broadcastAddress64 can be simplified.

This feature requires C++11 to be enabled. This isn't currently the case
(with Arduino 1.6.4), but is expected to be enabled in a future release.
A preprocessor check is used to ensure that the code compiles with and
without C++11.

Author: matthijskooijman

XBee.h

@@ -142,6 +142,20 @@
 #define PACKET_EXCEEDS_BYTE_ARRAY_LENGTH 2
 #define UNEXPECTED_START_BYTE 3
 
+/**
+ * C++11 introduced the constexpr as a hint to the compiler that things
+ * can be evaluated at compiletime. This can help to remove
+ * startup code for global objects, or otherwise help the compiler to
+ * optimize. Since the keyword is introduced in C++11, but supporting
+ * older compilers is a matter of removing the keyword, we use a macro
+ * for this.
+ */
+#if __cplusplus >= 201103L
+#define CONSTEXPR constexpr
+#else
+#define CONSTEXPR
+#endif
+
 /**
  * The super class of all XBee responses (RX packets)
  * Users should never attempt to create an instance of this class; instead
@@ -287,7 +301,7 @@ class XBeeResponse {
 
 class XBeeAddress {
 public:
-	XBeeAddress() {};
+	CONSTEXPR XBeeAddress() {};
 };
 
 /**
@@ -299,9 +313,9 @@ class XBeeAddress {
  */
 class XBeeAddress64 : public XBeeAddress {
 public:
-	XBeeAddress64(uint64_t addr) : _msb(addr >> 32), _lsb(addr) {}
-	XBeeAddress64(uint32_t msb, uint32_t lsb) : _msb(msb), _lsb(lsb) {}
-	XBeeAddress64() {}
+	CONSTEXPR XBeeAddress64(uint64_t addr) : _msb(addr >> 32), _lsb(addr) {}
+	CONSTEXPR XBeeAddress64(uint32_t msb, uint32_t lsb) : _msb(msb), _lsb(lsb) {}
+	CONSTEXPR XBeeAddress64() : _msb(0), _lsb(0) {}
 	uint32_t getMsb() {return _msb;}
 	uint32_t getLsb() {return _lsb;}
 	uint64_t get() {return (static_cast<uint64_t>(_msb) << 32) | _lsb;}