ExternalSource.cmake: external_git_repo(): Added support for an optional argument that allows the user to override the *_NAME and *_SRC_DIR variables set by the macro.
ExternalSource.cmake: external_git_repo(): Added support for an optional argument that allows the user to override the *_NAME and *_SRC_DIR variables set by the macro.

chaubold/buildem,janelia-flyem/buildem,chaubold/buildem,janelia-flyem/buildem,chaubold/buildem