clarify build instructions

As per discussion in #194, this PR makes the use of system-provided
dependencies the default method and moves the convenience scripts for compiling
software in the ``third-party`` directory to the ``scripts/`` directory. The
``scripts/`` directory is becoming increasingly misnamed but that's not the
focus of this PR.

Move the build-related information from README.md into a separate INSTALL.md
file to make it easier to find.

Addresses #131 by making the use of 'ninja' or 'make' entirely up to the person
building the source and not necessarily encouraging either by mandating use of
a Makefile. Also update the build information to give examples of using both.

The ongoing issue #154 may want to take note of this PR as it will affect the
homebrew script. Add a compatibility shim top-level Makefile so that homebrew
doesn't break for the moment. When #154 is closed, it may be time to remove it
for good.

Update the Travis build to use the convenience scripts but in their new
location.

Since the third-party compilation scripts now install everything to the
(non-hidden) ``third-party/dist``, update the build scripts to cope.