build: don't attempt to run config.guess on Windows

When cross-compiling LLVM to android from Windows (for LLVMSupport), we would
attempt to execute `config.guess` to determine the host triple since
`CMAKE_SYSTEM_NAME` is not Windows and `CMAKE_C_COMPILER` will be set to GNU or
Clang.  This will fail as `config.guess` is a shell script which cannot be
executed on Windows.  Simply log a warning instead.  The user can specify the
value for this instead in those cases.

llvm-svn: 363420