Shorten --enable-valgrind-testing to --enable-valgrind.

The former is too long.

Also removes a redundant check and combines the common valgrind
command line arguments. This adds --read-var-info=yes on MacOS.
It was disabled to work around a bug[1] but I don't see this issue
with valgrind 3.8.1, so I think it's safe to re-enable.

[1] http://comments.gmane.org/gmane.comp.debugging.valgrind.devel/2220