Work around problem reported by Steven M. Schweda in <http://lists.gnu.org/archive/html/bug-tar/2007-06/msg00002.html>: Tru64 5.1B with the Compaq compiler environment installed declares an 'isblank' function but does not define it in the C library. * lib/fnmatch.c (isblank): Check for HAVE_ISBLANK, too. * lib/regex_internal.h (isblank): Likewise. * m4/fnmatch.m4 (_AC_LIBOBJ_FNMATCH): Check for isblank existence. * m4/regex.m4 (gl_PREREQ_REGEX): Likewise.