Make BUILDTYPE=Release the default if no make parameters are specified

This probably simplifies the default case for most users of the
library, and reduces the risk of people accidentally trying to use
an unoptimized build.
Merge pull request #196 from mstorsjo/build-release-default

Make BUILDTYPE=Release the default if no make parameters are specified
Merge remote-tracking branch 'upstream/master' into travis.osx