Just discovered that $srcdir is available in the configure script for checking

Just discovered that $srcdir is available in the configure script for checking
for the existence of a file created by dist-hook. This lets us write a test
that knows the difference between a distribution tarball and something from the
repos. The building of the parsers and the manual can now be automatically
turned off in a make dist tarball.

git-svn-id: http://svn.complang.org/ragel/trunk@903 052ea7fc-9027-0410-9066-f65837a77df0