Report an error if neither S_ISREG nor

Report an error if neither S_ISREG nor
S_IFREG is defined, instead of using a test specific to glibc
2.2.  This should be safe, since POSIX requires S_ISREG and
Unix Version 7 had S_IFREG.  We don't need to check for
<sys/types.h> since we don't use any symbols that it defines.