cmake: allow for inclusion with CPM

Allow using backwards-cpp through CPM (Cmake Package
Manager)/FetchContent without using the ${BACKWARD_ENABLE} target

CPM (https://github.com/cpm-cmake/CPM.cmake) can now be used like so:

In a main/top-level CMakeLists.txt:

```cmake
include(cmake/CPM.cmake)

CPMAddPackage("gh:#1.7")
```

And in a project (or the same CMakeLists.txt):

```cmake
cmake_minimum_required(VERSION 3.9)

project (example VERSION 0.1)

add_executable(example
    main.cpp
)

target_link_libraries(example
    Backward::Backward
)
```

CMakeLists.txt

@@ -86,6 +86,11 @@ endif()
 add_library(backward ${libtype} backward.cpp)
 target_compile_definitions(backward PUBLIC ${BACKWARD_DEFINITIONS})
 target_include_directories(backward PUBLIC ${BACKWARD_INCLUDE_DIRS})
+if(BACKWARD_HAS_EXTERNAL_LIBRARIES)
+    set_target_properties(backward PROPERTIES
+                                   INTERFACE_LINK_LIBRARIES "${BACKWARD_LIBRARIES}")
+endif()
+add_library(Backward::Backward ALIAS backward)
 
 ###############################################################################
 # TESTS