Dictionary Fixes, 1.0 branch (2019.01.24.)

Recently the scripts were extended to produce .d files, which would be used by Ninja to detect when dictionaries need to be re-generated. As it turns out, the setup was not working correctly with Clang. Preventing the building of analysis releases on macOS.

This update only turns on the creation of the .d files if the Ninja generator was requested. And it fixes the incantation used for the .d file generation with Clang/GCC.

The update also makes sure that the dictionary generation is run in the runtime environment of the current project. In case it is the current project that sets up ROOT "for the first time".

Unfortunately Ninja with this setup is still overzealous with the re-generation of the dictionaries. (Always re-generating the dictionaries, even in no-op builds.) This still needs to be understood/fixed.