Replaced illegal tab in Makefile with spaces.

Replaced illegal tab in Makefile with spaces.

Commands, such as $(error ...), are not allowed to be indented with tabs
outside of targets, throwing an error instead of outputting the actual
error. The solution is to use innocuous spaces instead. Ideally, spaces
should be used everywhere outside targets, but since make does not mind
it if variable assignments are tab-indented outside targets, a complete
overhaul is not necessary. However, if more errors are added, it might
make more sense to be consistent.

Also, make will already add a period so I removed it.