These functions are in POSIX; hypot() is in C99. Also build cabs()
because it rides with hypot(), but don't declare cabs() in any header
file, because our compiler can't parse C99 "double complex" type.
Touch build.lua so it sees that .c files moved.
This undoes part of bfeb736, and returns to using DBL_MAX_EXP and
DBL_MIN_EXP from float.h.
Add a dependency on math/localmath.h and other local header files so
libc is rebuilt when those headers change.
