cfg.mk: Simplify backslash alignment check
cfg.mk: Simplify backslash alignment check

The use of [[:blank:]] was intended to cover tabs as well, but
it couldn't possibly work in its current form, so the regex was
tweaked in d09429abe826.

With the original reason for using [[:blank:]] now gone, we can
replace its usage with plain spaces. A comment about the purpose
of the check is added as well.

Signed-off-by: Andrea Bolognani <3ada0bee826c753786fdbba72243dfba997094cf@redhat.com>

jfehlig/libvirt,fabianfreyer/libvirt,eskultety/libvirt,nertpinx/libvirt,nertpinx/libvirt,jfehlig/libvirt,zippy2/libvirt,nertpinx/libvirt,crobinso/libvirt,nertpinx/libvirt,andreabolognani/libvirt,jfehlig/libvirt,crobinso/libvirt,crobinso/libvirt,zippy2/libvirt,andreabolognani/libvirt,zippy2/libvirt,libvirt/libvirt,eskultety/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,eskultety/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,crobinso/libvirt,eskultety/libvirt,olafhering/libvirt,olafhering/libvirt,andreabolognani/libvirt,zippy2/libvirt,olafhering/libvirt,jardasgit/libvirt,jardasgit/libvirt,andreabolognani/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,eskultety/libvirt,libvirt/libvirt,jardasgit/libvirt,libvirt/libvirt,jfehlig/libvirt,libvirt/libvirt,olafhering/libvirt