make CPM_DOWNLOAD_LOCATION overridable

Author: Arniiiii

test/transitive_dependency/cmake/CPM.cmake

@@ -1,16 +1,17 @@
-set(CPM_DOWNLOAD_VERSION 0.35.0)
-
-if(CPM_SOURCE_CACHE)
-  # Expand relative path. This is important if the provided path contains a tilde (~)
-  get_filename_component(CPM_SOURCE_CACHE ${CPM_SOURCE_CACHE} ABSOLUTE)
-  set(CPM_DOWNLOAD_LOCATION "${CPM_SOURCE_CACHE}/cpm/CPM_${CPM_DOWNLOAD_VERSION}.cmake")
-elseif(DEFINED ENV{CPM_SOURCE_CACHE})
-  set(CPM_DOWNLOAD_LOCATION "$ENV{CPM_SOURCE_CACHE}/cpm/CPM_${CPM_DOWNLOAD_VERSION}.cmake")
-else()
-  set(CPM_DOWNLOAD_LOCATION "${CMAKE_BINARY_DIR}/cmake/CPM_${CPM_DOWNLOAD_VERSION}.cmake")
-endif()
-
 if(NOT (EXISTS ${CPM_DOWNLOAD_LOCATION}))
+  
+  set(CPM_DOWNLOAD_VERSION 0.35.0)
+  
+  if(CPM_SOURCE_CACHE)
+    # Expand relative path. This is important if the provided path contains a tilde (~)
+    get_filename_component(CPM_SOURCE_CACHE ${CPM_SOURCE_CACHE} ABSOLUTE)
+    set(CPM_DOWNLOAD_LOCATION "${CPM_SOURCE_CACHE}/cpm/CPM_${CPM_DOWNLOAD_VERSION}.cmake")
+  elseif(DEFINED ENV{CPM_SOURCE_CACHE})
+    set(CPM_DOWNLOAD_LOCATION "$ENV{CPM_SOURCE_CACHE}/cpm/CPM_${CPM_DOWNLOAD_VERSION}.cmake")
+  else()
+    set(CPM_DOWNLOAD_LOCATION "${CMAKE_BINARY_DIR}/cmake/CPM_${CPM_DOWNLOAD_VERSION}.cmake")
+  endif()
+
   message(STATUS "Downloading CPM.cmake to ${CPM_DOWNLOAD_LOCATION}")
   file(DOWNLOAD
        https://github.com/TheLartians/CPM.cmake/releases/download/v${CPM_DOWNLOAD_VERSION}/CPM.cmake