regcomp.c: Slight refactor of [[:blank:]]

regcomp.c: Slight refactor of [[:blank:]]

The C library function isblank() is not available on every platform.  A
Configure probe determines if it exists on the current platform.  If so,
compiling it can be done just like all the similar ones.  Only if
isblank() is not present does there need to be special handling.

This commit changes the cases of a switch statement so that if isblank()
is present, there is no special handling.