Mark CPM_SOURCE_CACHE as a CACHE variable

By omitting the variable CPM_SOURCE_CACHE as CACHE'd, it is actually
given to CPM, but does not appear when viewing it's content, e.g.,
through ccmake. Mark is as CACHE so that it will also appear.

CMakeLists.txt

@@ -32,7 +32,7 @@ set (OPENAIR_DIR     ${CMAKE_SOURCE_DIR})
 
 include("cmake_targets/macros.cmake")
 if(NOT DEFINED ENV{CPM_SOURCE_CACHE})
- set(CPM_SOURCE_CACHE ~/.cache/cpm/)
+  set(CPM_SOURCE_CACHE ~/.cache/cpm/ CACHE FILEPATH "Directory to download CPM dependencies")
 endif()
 include("cmake_targets/CPM.cmake")