* Install sources starting with '/' are absolute.
    * Install sources starting with '$(' are explicitly relative to that prefix variable.
    * Other install source remain relative to $(SOURCE_PREFIX).
    * LOCAL_REPO_SOURCES_DIR varaible is no longer used, use $(LOCAL_REPO_SYNC_DIR) instead.
    * Version 0.9.18.1
merge from master
* Added some variable docs
* Moved the curl files to $(LOCAL_REPO_BOOKKEEPING_DIR)/curl instead of $(LOCAL_REPO_SYNC_DIR)/curl
 * It's possible now to install sources from any folder regardless of OS. This mainly enables the uses
        of Common folder in the same level of Win and Mac.