BUILD: compiler: rename __unreachable() to my_unreachable()

Olivier reported that on FreeBSD __unreachable is already defined
and causes build warnings. Let's rename it then.

include/common/compiler.h

@@ -89,9 +89,9 @@
  * below was introduced in gcc 4.5, and before it we didn't care.
  */
 #if __GNUC__ >= 5 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 5)
-#define __unreachable() __builtin_unreachable()
+#define my_unreachable() __builtin_unreachable()
 #else
-#define __unreachable()
+#define my_unreachable()
 #endif
 
 /*

src/hlua.c

@@ -68,7 +68,7 @@
  *   MAY_LJMP() marks an lua function that may use longjmp.
  */
 #define __LJMP
-#define WILL_LJMP(func) do { func; __unreachable(); } while(0)
+#define WILL_LJMP(func) do { func; my_unreachable(); } while(0)
 #define MAY_LJMP(func) func
 
 /* This couple of function executes securely some Lua calls outside of