memtest: Add missing free()

Closes #1614
Remove redundant event loop fd processing

Reason:
https://github.com/antirez/redis/pull/1450#issuecomment-30432925

Closes #1450
Improve accuracy of HAVE_ATOMIC dependency check

[I had to split out the clang check due to
 clang *really* not liking the __GLIBC_PREREQ macro; -matt]

Closes #1456