Document how to build from source with full debuginfo

[lrowe]

The debug build includes only partial debuginfo to save space but this makes gdb info args and info locals error with No symbol table info available.

There is probably a better way of doing this using $GN_ARGS but I am not familiar with GN and was not able to make it work so took the following approach.

• Clone rusty_v8 and comment out three lines in .gn.

  # Minimize size of debuginfo in distributed static library.
  #line_tables_only = true
  #no_inline_line_tables = true
  #symbol_level = 1
  use_debug_fission = false. # Always want this one

• Build librusty_v8.a with V8_FROM_SOURCE=1 cargo build -vv.

• Build own project with RUSTY_V8_ARCHIVE=/path/to/rusty_v8/target/debug/gn_out/obj/librusty_v8.a V8_FORCE_DEBUG=1 cargo build

The static lib with full debuginfo is 3.6GB so it wouldn't make sense to change the default here, but it would be great if it could be made a one liner.