Enable OS X building in travis-ci
Ignore build products in xilinx/
Remove unused VGA test code
Absorb solidus into BIN definition

This permits the command-line override of `BIN=` (empty) to look up the
commands in $PATH.
Rewrite inappropriate variable into rule

Also, refrain from running rmdir with no args
Reorder rules to remedy `make -B` issue

Before this change, `make -B clock.texe` would fail because it would use the
%.tas rule from Makefile.rules instead of the %.tas.cpp rule from this
Makefile. Reordering permits `-B` to work.
Combine .ucf files for simplicity
Refactoring build rules to hoist options
Remove needless atomic moves

A badly-written Make rule could leave a file half-constructed or try to
consume it before it was finished being built (in a parallel make), which was
why we incrementally built some files to `$@.$$$$` and then moved to `$@` at
the end. These rules were not badly constructed, though, and testing by
inserting sleeps and killing the in-progress construction showed Make properly
disposing of the half-made file.
Improve delegated make rules
Keep only non-default options in .xst

Using the .xrpt file, removed default options thus :

	grep DEFAULT= Tenyr_xst.xrpt | \
	perl -lne '($d) = /DEFAULT="(.*?)"/;' \
	       -e '($v) = /value="(.*?)"/;'   \
	       -e '($l) = /label="(.*?)"/;'   \
	       -e 'print "$l\\>" if uc $d ne uc $v' > keep
	(grep -v '^-' Tenyr.xst ; grep -f keep Tenyr.xst) > temp
	mv temp Tenyr.xst
Manually remove needless .xst options
Permit optional higher optimisation
Stop using (naughty) .PRECIOUS
Lower generic rule
Simplify data2mem rule appearance
Remove obsoleted ignores
Update location of filter.filter ignore
Generalise building across OSes

Previously, building on a new OS required first creating a new
Makefile.$(OS) and src/os/$(OS) directory. Now UNIX-likes should build
automatically. FreeBSD builds without a special Makefile.
Stop depending on gcc