A few fixes to atsmake

In general, the construction:

ifeq ("$(VAR)","VALUE")
...

doesn't work as expected; expansion of variables with whitespace can
cause problems. The solution is to do:

ifeq ($(strip $(VAR),VALUE))
...

which strips off any extra whitespace.

In the case where VALUE is the empty string, it makes more sense to
write:

ifndef VAR
...

Which results in many instances of the construction:

ifndef ...
else
...
endif

I changed this to:

ifdef
...
endif

I also removed the initial empty declarations of SOURCES_?ATS in
atsmake-pre.mk; This allows for the use of ifdef.

for more info, see:

https://gnu.org/software/make/manual/make.html#Conditional-Syntax
Merge pull request #41 from bbarker/master

minor update to readmes
Merge pull request #42 from zenhack/master

A few fixes to atsmake
Updating: minorly improving
Updating: very minorly
A few more atsmake fixes

More converting ifeq -> ifndef and the like; looks like I missed a
couple before.
Merge pull request #43 from zenhack/master

A few more atsmake fixes
Updating: minor changes.
Updating: minor changes.
Updating: tweaking a bit
Updating: keeping an example up-to-date