Add per package CPM_DOWNLOAD controls (#336)

* Add per project CPM_DOWNLOAD controls

Introduces support for `CPM_<PACKAGE>_DOWNLOAD` variable ( and env )
which allows finer grained control.

* Address PR feedback on API names

README.md

@@ -183,6 +183,7 @@ You can use `CPM_SOURCE_CACHE` on GitHub Actions workflows [cache](https://githu
 If set, CPM will forward all calls to `CPMFindPackage` as `CPMAddPackage`.
 This is useful to create reproducible builds or to determine if the source parameters have all been set correctly.
 This can also be set as an environmental variable.
+This can be controlled on a per package basis with the `CPM_DOWNLOAD_<dependency name>` variable.
 
 ### CPM_USE_LOCAL_PACKAGES
 

cmake/CPM.cmake

@@ -251,7 +251,13 @@ function(CPMFindPackage)
     endif()
   endif()
 
-  if(CPM_DOWNLOAD_ALL)
+  set(downloadPackage ${CPM_DOWNLOAD_ALL})
+  if(DEFINED CPM_DOWNLOAD_${CPM_ARGS_NAME})
+    set(downloadPackage ${CPM_DOWNLOAD_${CPM_ARGS_NAME}})
+  elseif(DEFINED ENV{CPM_DOWNLOAD_${CPM_ARGS_NAME}})
+    set(downloadPackage $ENV{CPM_DOWNLOAD_${CPM_ARGS_NAME}})
+  endif()
+  if(downloadPackage)
     CPMAddPackage(${ARGN})
     cpm_export_variables(${CPM_ARGS_NAME})
     return()