cmake: fix rpath for macos dynamic linkage

src/CMakeLists.txt

 target_link_libraries(
   espeak-ng-bin PRIVATE espeak-ng espeak-ng-config
 )
+set_target_properties(
+  espeak-ng-bin PROPERTIES
+  MACOSX_RPATH ON
+  INSTALL_RPATH "${CMAKE_INSTALL_PREFIX}/lib"
+)
 if (MINGW)
   target_link_options(espeak-ng-bin PRIVATE "-static-libstdc++" "-static")
 endif()

src/libespeak-ng/CMakeLists.txt

 
   espeak_api.c
 )
+set_target_properties(
+  espeak-ng PROPERTIES
+  MACOSX_RPATH ON
+  INSTALL_RPATH "${CMAKE_INSTALL_PREFIX}/lib"
+)
 
 target_include_directories(espeak-ng BEFORE PRIVATE $<TARGET_PROPERTY:espeak-include,INTERFACE_INCLUDE_DIRECTORIES>)