libragephoto: rename BINDING to PUBLIC, some other changes

- doc/Doxyfile.in: rename BINDING to PUBLIC
- doc/usage.doc: fix a usage example
- CMakeLists.txt: include CPack only when libragephoto is Top Level
- RagePhoto.h: rename BINDING TO PUBLIC
- RagePhotoC.h: rename BINDING TO PUBLIC
- RagePhotoLibrary.h: rename BINDING to PUBLIC, add new checks

doc/Doxyfile.in

@@ -12,5 +12,5 @@ EXPAND_ONLY_PREDEF     = YES
 PREDEFINED             = "__cplusplus=201703L" \
                          "RAGEPHOTO_CXX_STD=17" \
                          "@LIBRAGEPHOTO_API@" \
-                         "LIBRAGEPHOTO_C_BINDING=" \
-                         "LIBRAGEPHOTO_CXX_BINDING="
+                         "LIBRAGEPHOTO_C_PUBLIC=" \
+                         "LIBRAGEPHOTO_CXX_PUBLIC="

doc/usage.doc

@@ -96,7 +96,7 @@ bool saveJpeg(RagePhoto* ragePhoto, const std::string& filename) {
 }
 
 // Using the saveJpeg function
-const char* filename = "PGTA5123456789";
+const char* filename = "PGTA5123456789.jpg";
 const bool saved = saveJpeg(&ragePhoto, filename);
 \endcode