Add dependencies to generating version.h in the object files that use it

Previously the makefile didn't know that there was any dependency between
compiling welsDecoderExt.o and welsEncoderExt.o and the regeneration
of the version.h header. This meant that in parallel builds (make -jX),
make could try to regenerate version.h while compiling welsDecoderExt.o
and welsEncoderExt.o, which would lead to errors about version.h not
existing.

Also add some spacing around the makefile rules.

This fixes parallel make building in certain cases.
Fix the name of the version generation makefile target
Merge pull request #1206 from mstorsjo/version-generate-deps

Fix generating version.h in parallel make builds (make -jX)