regcomp.c: Properly handle no isblank(), isascii()

regcomp.c: Properly handle no isblank(), isascii()

Configure probes whether or not these two C library functions are
present or not.  (However until commit
1c6eef9acffe4b512210edba79119e423ea4874a it didn't find isblank() even
if present.)

However the code changed by this commit always presumed both functions
were present.  That there were no failure reports from the field
indicates that Perl is being run on systems where they are present.