doc/Makefile: Fix rst2html detection
Displaying the output from the failing call to "which" didn't prevent
make from doing stupid things later. We now only search for "rst2html"
and fallback to "rst2html.py". If neither name is found, make will
eventually abort when we try to use $(RST2HTML).
Basic install:
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
See http://mercurial.selenic.com/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.