Fix bug in GonioPhotometricLight::Scale() method.

The SPECTRUM_ILLUMINANT enum was being passed to MIPMap::Lookup()
rather than to the Spectrum() constructor!  This in turn caused
the lookup method to do a wide trilinear filter, thus basically
completely breaking gontiophotometric lights.
Fix bug in filename handling in dipole subsurface renderer.

Previously, we were using ParamSet::FindOneString() to get the
filename from the parameters; we should have been using
FindOneFilename(); this in turn led to a case where if you rendered
pbrt-scenes/ballpile.pbrt from a different directory, the
"surfacepoints.out" file would be written to the directory that
ballpile.pbrt was in, but then it wouldn't be found by the
DipoleSubsurfaceIntegrator when it went to render the scene..
fix rare error where make clobbers needed intermediate files (Zach Devito)
Fix naming of variable to be consistent with derivation
Fix bug in AnimatedTransform::MotionBounds (Yunjian Ding)
Merge branch 'master' of github.com:mmp/pbrt-v2
Merge branch 'master' of ssh://home/Users/mmp/git/pbrt
Fix formatting in error strings for negative/infinite luminances.
Fix accel.d dtrace script
Add flag to silence slew of compiler warnings on OSX 10.9
Add #error directive if g++ 4.8.2 is used (Thanks @BachiLi!)
Merge branch 'master' of github.com:mmp/pbrt-v2