Implement support a CMKR_CACHE environment variable

Closes #17

cmake/cmkr.cmake

@@ -53,18 +53,45 @@ else()
 endif()
 
 # Use cached cmkr if found
-set(CMKR_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}/_cmkr_${CMKR_TAG}")
+if(DEFINED ENV{CMKR_CACHE} AND EXISTS "$ENV{CMKR_CACHE}")
+    set(CMKR_DIRECTORY_PREFIX "$ENV{CMKR_CACHE}")
+    string(REPLACE "\\" "/" CMKR_DIRECTORY_PREFIX "${CMKR_DIRECTORY_PREFIX}")
+    if(NOT CMKR_DIRECTORY_PREFIX MATCHES "\\/$")
+        set(CMKR_DIRECTORY_PREFIX "${CMKR_DIRECTORY_PREFIX}/")
+    endif()
+else()
+    set(CMKR_DIRECTORY_PREFIX "${CMAKE_CURRENT_BINARY_DIR}/_cmkr_")
+endif()
+set(CMKR_DIRECTORY "${CMKR_DIRECTORY_PREFIX}${CMKR_TAG}")
 set(CMKR_CACHED_EXECUTABLE "${CMKR_DIRECTORY}/bin/${CMKR_EXECUTABLE_NAME}")
 
-if(NOT CMKR_CACHED_EXECUTABLE STREQUAL CMKR_EXECUTABLE AND CMKR_EXECUTABLE MATCHES "^${CMAKE_CURRENT_BINARY_DIR}/_cmkr")
+# Handle upgrading logic
+if(CMKR_EXECUTABLE AND NOT CMKR_CACHED_EXECUTABLE STREQUAL CMKR_EXECUTABLE)    
+    if(CMKR_EXECUTABLE MATCHES "^${CMAKE_CURRENT_BINARY_DIR}/_cmkr")
+        if(DEFINED ENV{CMKR_CACHE} AND EXISTS "$ENV{CMKR_CACHE}")
+            message(AUTHOR_WARNING "[cmkr] Switching to cached cmkr: '${CMKR_CACHED_EXECUTABLE}'")
+            if(EXISTS "${CMKR_CACHED_EXECUTABLE}")
+                set(CMKR_EXECUTABLE "${CMKR_CACHED_EXECUTABLE}" CACHE FILEPATH "Full path to cmkr executable" FORCE)
+            else()
+                unset(CMKR_EXECUTABLE CACHE)
+            endif()
+        else()
             message(AUTHOR_WARNING "[cmkr] Upgrading '${CMKR_EXECUTABLE}' to '${CMKR_CACHED_EXECUTABLE}'")
             unset(CMKR_EXECUTABLE CACHE)
+        endif()
+    elseif(DEFINED ENV{CMKR_CACHE} AND EXISTS "$ENV{CMKR_CACHE}" AND CMKR_EXECUTABLE MATCHES "^${CMKR_DIRECTORY_PREFIX}")
+        message(AUTHOR_WARNING "[cmkr] Upgrading cached '${CMKR_EXECUTABLE}' to '${CMKR_CACHED_EXECUTABLE}'")
+        unset(CMKR_EXECUTABLE CACHE)
+    endif()
 endif()
 
 if(CMKR_EXECUTABLE AND EXISTS "${CMKR_EXECUTABLE}")
     message(VERBOSE "[cmkr] Found cmkr: '${CMKR_EXECUTABLE}'")
 elseif(CMKR_EXECUTABLE AND NOT CMKR_EXECUTABLE STREQUAL CMKR_CACHED_EXECUTABLE)
     message(FATAL_ERROR "[cmkr] '${CMKR_EXECUTABLE}' not found")
+elseif(NOT CMKR_EXECUTABLE AND EXISTS "${CMKR_CACHED_EXECUTABLE}")
+    set(CMKR_EXECUTABLE "${CMKR_CACHED_EXECUTABLE}" CACHE FILEPATH "Full path to cmkr executable" FORCE)
+    message(STATUS "[cmkr] Found cached cmkr: '${CMKR_EXECUTABLE}'")
 else()
     set(CMKR_EXECUTABLE "${CMKR_CACHED_EXECUTABLE}" CACHE FILEPATH "Full path to cmkr executable" FORCE)
     message(VERBOSE "[cmkr] Bootstrapping '${CMKR_EXECUTABLE}'")