Arrange to compile flex output files as inclusions into other files

Arrange to compile flex output files as inclusions into other files
(usually bison output files), not as standalone files.  This hack
works around flex's insistence on including <stdio.h> before we are
able to include postgres.h; postgres.h will already be read before
the compiler starts to read the flex output file.  Needed for largefile
support on some platforms.