Add jerryscript-compiler.h public header to cover compiler incompatibilities (#2313)

In general, public headers should not have compiler-specific
constructs but both the core and the port headers have attributes,
which are non-standard. It's better to factor out such constructs
to a common place (a new header) and hide them behind macros, which
can then be defined on a per-compiler basis.

This patch moves the existing definitions of function attributes and
likely/unlikely builtins to the new header. At the same time, it
unifies the names of these attribute defines and where they are
used. Moreover, it touches on jerry-main and removes the uses of
`__attribute__((unused))` entirely and replaces them with the
elsewhere used `(void) ...` pattern.

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu

jerry-core/include/jerryscript-core.h

@@ -20,18 +20,13 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#include "jerryscript-compiler.h"
+
 #ifdef __cplusplus
 extern "C"
 {
 #endif /* __cplusplus */
 
-#ifdef __GNUC__
-#define JERRY_DEPRECATED_API __attribute__((deprecated))
-#else /* !__GNUC__ */
-/* TODO: for other compilers */
-#define JERRY_DEPRECATED_API
-#endif /* __GNUC__ */
-
 /** \addtogroup jerry Jerry engine interface
  * @{
  */
@@ -491,9 +486,9 @@ jerry_value_t jerry_get_object_keys (const jerry_value_t obj_val);
 jerry_value_t jerry_get_prototype (const jerry_value_t obj_val);
 jerry_value_t jerry_set_prototype (const jerry_value_t obj_val, const jerry_value_t proto_obj_val);
 
-JERRY_DEPRECATED_API
+JERRY_ATTR_DEPRECATED
 bool jerry_get_object_native_handle (const jerry_value_t obj_val, uintptr_t *out_handle_p);
-JERRY_DEPRECATED_API
+JERRY_ATTR_DEPRECATED
 void jerry_set_object_native_handle (const jerry_value_t obj_val, uintptr_t handle_p,
                                      jerry_object_free_callback_t freecb_p);