• Belatedly update `graphics/embree' to the latest version, 2.17.1
• Add ISPC option and enable it by default (this pulls rather heavy `devel/llvm50' via BUILD_DEPENDS and normally would not be enabled by default, but since Embree is most likely to be used in graphics applications, it is safe to assume that users already have LLVM 5 installed because X.org/Mesa stack unconditionally requires it)
• OpenEXR support was removed upstream since version 2.13.0 back in November 11, 2016: https://github.com/embree/embree/commit/483491e (as usual, unfortunately, no explanation was provided in the log)
• Remove CXXFLAGS+=-msse2 required for i386 earlier, not needed now
• Add missing USE_GL, USE_XORG components as prompted by `stage-qa'

PR: 214577, 224066

Because GitHub releases (tarballs) are not fetched with correct modification
time, set TIMESTAMP to 1510648523 which corresponds to commit 90e49f2 tagged
as this release.