See Implementation-defined behavior in The C Preprocessor, for details of these aspects of implementation-defined behavior.
#pragmadirectives where header name preprocessing tokens are recognized (C11 6.4, C11 6.4.7).
#includedirective are combined into a header name (C90 6.8.2, C99 and C11 6.10.2).
#includeprocessing (C90 6.8.2, C99 and C11 6.10.2).
STDC #pragmadirective (C90 6.8.6, C99 and C11 6.10.6).
__TIME__when respectively, the date and time of translation are not available (C90 6.8.8, C99 6.10.8, C11 22.214.171.124).