$ flex src/scan-skel.l
src/scan-skel.l:145: multiple <<EOF>> rules for start condition SC_AT_DIRECTIVE_ARGS
src/scan-skel.l:145: multiple <<EOF>> rules for start condition SC_AT_DIRECTIVE_SKIP_WS
This is warning, and it seems there are no means to make it an error.
* src/scan-skel.l: Restore the start-condition INITIAL for an <<EOF>>
clause.