use malloc compiler attribute if available

This informs the compiler that these functions return allocated
memory.  That allows it to make some optimization decisions which
can produce better code.

src/gd.h

@@ -65,6 +65,7 @@ extern "C" {
 # endif
 # define BGD_STDCALL __stdcall
 # define BGD_EXPORT_DATA_IMPL
+# define BGD_MALLOC
 #else
 # if defined(__GNUC__) || defined(__clang__)
 #  define BGD_EXPORT_DATA_PROT __attribute__ ((__visibility__ ("default")))
@@ -74,6 +75,7 @@ extern "C" {
 #  define BGD_EXPORT_DATA_IMPL
 # endif
 # define BGD_STDCALL
+# define BGD_MALLOC __attribute__ ((__malloc__))
 #endif
 
 #define BGD_DECLARE(rt) BGD_EXPORT_DATA_PROT rt BGD_STDCALL

src/gdhelpers.h

@@ -20,8 +20,8 @@ extern "C" {
 		in gd.h, where callers can utilize it to correctly
 		free memory allocated by these functions with the
 		right version of free(). */
-	void *gdCalloc (size_t nmemb, size_t size);
-	void *gdMalloc (size_t size);
+	void *gdCalloc(size_t nmemb, size_t size) BGD_MALLOC;
+	void *gdMalloc(size_t size) BGD_MALLOC;
 	void *gdRealloc (void *ptr, size_t size);
 	/* The extended version of gdReallocEx will free *ptr if the
 	 * realloc fails */