don't die on V=$whatever
add/change entire string.h and parts of libc++
add more libc/libc++ fragments
add more libc$anything
add a memory allocator and <new>
add lots of c++ headers, some c headers too

 * <cerrno>
 * <cfloat>
 * <cstdarg>
 * <__debug>
 * <errno.h>
 * <__functional_base>
 * <iosfwd>, much reduced (only forwards decls for string)
 * <iterator>
 * <numeric>
 * <__refstring>
 * <tuple>
 * <typeindex>
 * <typeinfo>
correctly run the preprocessor over C files for deps