Use current source paths for config file creation command

That way, the project integrate more nicely when used as a cmake sub-project.

CMakeLists.txt

@@ -31,7 +31,7 @@ find_package(Perl)
 if(PERL_FOUND)
 
     # If NULL Entropy is configured, display an appropriate warning
-    execute_process(COMMAND ${PERL_EXECUTABLE} ${CMAKE_SOURCE_DIR}/scripts/config.pl -f ${CMAKE_SOURCE_DIR}/include/mbedtls/config.h get MBEDTLS_TEST_NULL_ENTROPY
+    execute_process(COMMAND ${PERL_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/scripts/config.pl -f ${CMAKE_CURRENT_SOURCE_DIR}/include/mbedtls/config.h get MBEDTLS_TEST_NULL_ENTROPY
                         RESULT_VARIABLE result)
     if(${result} EQUAL 0)
         message(WARNING ${NULL_ENTROPY_WARNING})