Fix setting default CPM source cache directory

CPM kept overriding this with the helpful value `"OFF"`. No more, CPM, no more.

Signed-off-by: Till Schneidereit <till@tillschneidereit.net>

Author: tschneidereit

CMakeLists.txt

@@ -21,8 +21,8 @@ endif()
 message(STATUS "Using host API: ${HOST_API}")
 
 # Ensure that the CPM cache is created outside the build dir, even if no location is specified by the developer.
-if(NOT DEFINED ENV{CPM_SOURCE_CACHE} AND NOT DEFINED CPM_SOURCE_CACHE)
-    set(CPM_SOURCE_CACHE ${CMAKE_CURRENT_SOURCE_DIR}/deps/cpm_cache)
+if(NOT DEFINED ENV{CPM_SOURCE_CACHE})
+    set(ENV{CPM_SOURCE_CACHE} ${CMAKE_CURRENT_SOURCE_DIR}/deps/cpm_cache)
 endif()
 include("CPM")
 include("toolchain")