Support CPM_SOURCE_CACHE on windows (#109)

* use semicolon as empty command and add test

* remove platform specific test coverage

* only run actions for pushes and prs into master (avoid duplication)

* remove cache test as it's already part of the test suite

* update version

* test cache reuse

test/unit/cache.cmake

@@ -8,6 +8,7 @@ set(CPM_SOURCE_CACHE_DIR "${CMAKE_CURRENT_BINARY_DIR}/CPM")
 set(TEST_BUILD_DIR ${CMAKE_CURRENT_BINARY_DIR}/cache)
 
 function(clear_cache)
+  message(STATUS "clearing CPM cache")
   FILE(REMOVE_RECURSE ${CPM_SOURCE_CACHE_DIR})
   
   if (EXISTS "${CPM_SOURCE_CACHE_DIR}")
@@ -104,6 +105,16 @@ if (NOT EXISTS "${CPM_SOURCE_CACHE_DIR}/fibonacci")
   ASSERTION_FAILED("fibonacci not in cache")
 endif()
 
+## Reuse cached packages for other build
+
+execute_process(
+  COMMAND 
+  ${CMAKE_COMMAND} -E env "CPM_SOURCE_CACHE=${CPM_SOURCE_CACHE_DIR}" ${CMAKE_COMMAND} "-H${CMAKE_CURRENT_LIST_DIR}/cache" "-B${TEST_BUILD_DIR}-2"
+  RESULT_VARIABLE ret
+)
+
+ASSERT_EQUAL(${ret} "0")
+
 ## Overwrite CPM_SOURCE_CACHE with argument
 
 reset_test()