bootstrap: Add Clang to known compiler candidates

Add `clang` and `clang++` after the corresponding `gcc` and `g++`
candidates.  Otherwise it is not tried on platforms where we do not
enumerate the toolchain pair candidates.

bootstrap

@@ -220,8 +220,8 @@ cmake_doc_dir_default="`cmake_install_dest_default DOC ${cmake_doc_dir_keyword}`
 cmake_man_dir_default="`cmake_install_dest_default MAN ${cmake_man_dir_keyword}`"
 cmake_xdgdata_dir_default="`cmake_install_dest_default XDGDATA ${cmake_xdgdata_dir_keyword}`"
 
-CMAKE_KNOWN_C_COMPILERS="cc gcc xlc icc tcc"
-CMAKE_KNOWN_CXX_COMPILERS="aCC xlC CC g++ c++ icc como "
+CMAKE_KNOWN_C_COMPILERS="cc gcc clang xlc icc tcc"
+CMAKE_KNOWN_CXX_COMPILERS="aCC xlC CC g++ clang++ c++ icc como "
 CMAKE_KNOWN_MAKE_PROCESSORS="gmake make"
 
 CMAKE_PROBLEMATIC_FILES="\