Use PROJECT_SOURCE_DIR instead of CMAKE_SOURCE_DIR for includes (census-instrumentation#267)

Allows including the project using `add_subdirectory` command.

`CMAKE_SOURCE_DIR` always resolves to the top level project, breaking includes.

Author: nvartolomei

cmake/OpenCensusHelpers.cmake

@@ -47,11 +47,11 @@ function(opencensus_lib NAME)
   if(ARG_SRCS)
     add_library(${_NAME} ${ARG_SRCS})
     target_link_libraries(${_NAME} PUBLIC ${ARG_DEPS})
-    target_include_directories(${_NAME} PUBLIC ${CMAKE_SOURCE_DIR})
+    target_include_directories(${_NAME} PUBLIC ${PROJECT_SOURCE_DIR})
   else()
     add_library(${_NAME} INTERFACE)
     target_link_libraries(${_NAME} INTERFACE ${ARG_DEPS})
-    target_include_directories(${_NAME} INTERFACE ${CMAKE_SOURCE_DIR})
+    target_include_directories(${_NAME} INTERFACE ${PROJECT_SOURCE_DIR})
   endif()
   if(ARG_PUBLIC)
     add_library(${PROJECT_NAME}::${NAME} ALIAS ${_NAME})