Skip fetch only works when CPM_SOURCE_CACHE is defined

[ScottBailey]

When the environment variable CPM_SOURCE_CACHE is set, re-running cmake does not cause a fetch and patch.

When unset, fetch and patch both occur.

The problem observed (see #572 for more discussion) is that the unset path fetch does not overwrite the source and so the patch command fails.

#572 suggests a fix that could ignore errors during patching. I think this could be an issue when a user is testing version updates; it would be nice if we just, say, updated package A to latest version and running CMake told us the patches were incompatible/not applied. I don't think -t reports that?

In my testing, I see that when the environment variable is set, we skip the fetch and patch steps. Maybe we can add a cache variable to indicate the initial fetch and patch skips were successful?

Recreation of this is relatively simple, see https://github.com/ScottBailey/cpm-cmake-test-577 for an automated working example.

@lemire

[wolfpld]

A simple solution seems to be to always enable the cache.

--- a/cmake/CPM.cmake
+++ b/cmake/CPM.cmake
@@ -154,7 +154,7 @@ set(CPM_DRY_RUN
 if(DEFINED ENV{CPM_SOURCE_CACHE})
   set(CPM_SOURCE_CACHE_DEFAULT $ENV{CPM_SOURCE_CACHE})
 else()
-  set(CPM_SOURCE_CACHE_DEFAULT OFF)
+  set(CPM_SOURCE_CACHE_DEFAULT ${CMAKE_CURRENT_BINARY_DIR}/.cpm-cache)
 endif()
 
 set(CPM_SOURCE_CACHE

[lemire]

Intriguing.