-#TODO: add check that source doesn't contain any untracked files
-if(NOT CPACK_INSTALL_CMAKE_PROJECTS) #building source package
+set(BUILDING_SOURCE_PACKAGE OFF)
+# The essential difference in building a source package is that install() rules
+# from the CMake project are not considered when deciding what to package.
+# Instead, only the listed directories are packaged (and the listed directories
+# contain the source tree).
+if (NOT CPACK_INSTALL_CMAKE_PROJECTS)
+ set(BUILDING_SOURCE_PACKAGE ON)
+endif()
+
+if (BUILDING_SOURCE_PACKAGE)
+ # TODO: add check that source doesn't contain any untracked files