Work around problem reported by Steven M. Schweda in

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.