maketx bug in fix_latl_edges -- apparent cut/paste error summed incorrectly.

This is the special routine that patches up the poles, only comes into
play for OpenEXR environment maps.  The top and/or bottom rows are supposed
to be averaged, which happens by summing, then dividing by the number of
pixels summed.  Somehow, one extra pixel got added, I can only imagine that
it was a cut-and-paste error.  But the result is that OpenEXR environment
maps had just a teensy bit too much energy at the poles.  But this compounds
at each MIP level, so for extremely blurry environment lookups, it could
look much brighter than it should have.

It was clearly wrong before, right now.  But beware -- if you re-convert
an environment map, this could change its appearance.  (Only if it's an
OpenEXR env map, it did not affect ordinary texture, nor TIFF
environment maps.)
New build flags: OIIO_BUILD_TOOLS, OIIO_BUILD_TESTS
Both default to ON, but when turned off will skip building the command-line
tools (oiiotool, maketx, etc.) and tests, respectively.  This can speed up
builds if you know for sure that all you want the bare libOpenImageIO.
Fixes #276