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.

Note --read-var-info=yes is disabled on MacOS work around a bug.
http://comments.gmane.org/gmane.comp.debugging.valgrind.devel/2220
Merge pull request #16 from rillian/dev-fixes

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