Update cmake

[oliverbunting]

Extends the cmake file for easier inclusion in other-projects. The PR supports both embedded:

add_subdirectory(Parser-SPEF)
target_link_library(my_target PRIVATE Parser-SPEF)

or find_package

find_package(Parser-SPEF 3.9 REQUIRED)
target_link_library(my_target PRIVATE Parser-SPEF::Parser-SPEF)