[Predef-contribute] xlclang

[Daniel S. <da...@ha...>]

Hey,

IBM obviously thought it is a good idea to provide a compiler that looks 
almost like clang but doesn't work like clang. It just cause a bug in curl 
[1]. xlclang's predefined macros are in [2] on page 618.

*both* xlc and clang macros seem to be there.

[1] = https://github.com/curl/curl/issues/3474
[2] = 
https://www.ibm.com/support/knowledgecenter/SSGH3R_16.1.0/com.ibm.compilers.aix.doc/compiler.pdf?view=kc

-- 

  / daniel.haxx.se