Source files can have properties that affect how they are built.

set_source_files_properties([file1 [file2 [...]]]
                            PROPERTIES prop1 value1
                            [prop2 value2 [...]])

Set properties associated with source files using a key/value paired list. See Properties on Source Files for the list of properties known to CMake. Source file properties are visible only to targets added in the same directory (CMakeLists.txt).