* Update to 2018.3 and improve version URLs, dependencies and patching.
Improve to version -> URL translation to be algorithmic rather than manual.
Make binutils build dependency explicit where appropriate because we're sensitive to the assembler used.
Build's use of '-mrtm' is only patched away if it needs to be because the assembler is too old.
Incorporate @adamjstewart request to convert GCC version check to a conflict.
* Allow specification of C++ standard.
* Improve TBB patch per PR comments.
* Restrict RTM patch application to older OS only.
* New version 2018_U4.
