decouple third-party directory from build system unless desired

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.