Master cmake clang build

[iains]

two patchlets for clang support.

1/ avoid the need to install the empty cody-config.hh when the compiler supports __has_include (which is most likely all compilers that can build the code). This avoids the need to generate the empty header in the cmake configuration (we can revisit if the header becomes non-empty).

2/ Although the library was building in the LLVM tree for "make [all]", the dependencies were not correct when doing "make clang" with the library as a required sub-dependency of a clang constituent library. Fixed by making the build explicit as 'stand alone' and otherwise presuming it to be an LLVM-based configuration (which is the intent of the cmake implementation).

[urnathan]

the github UI is fucking crap. approving a single patch wants to merge the whol