cmake: add default data path from CMAKE_INSTALL_PREFIX

src/libespeak-ng/CMakeLists.txt

@@ -69,6 +69,8 @@ if (NOT BUILD_SHARED_LIBS)
   target_compile_definitions(espeak-ng INTERFACE "LIBESPEAK_NG_EXPORT=1")
 endif()
 
+target_compile_definitions(espeak-ng PRIVATE "PATH_ESPEAK_DATA=\"${CMAKE_INSTALL_PREFIX}/share/espeak-ng-data\"")
+
 if (USE_ASYNC)
   target_sources(espeak-ng PRIVATE
     event.c