change LYAML_SRC to LYAML_ROOT, which points to the top source
directory now.
LYAML_LIB and (new) LYAML_LUA_DIR are based relative to that
directory.
There's no install step at all now--the static library and the .lua
files are now used directly from the source tree.
Signed-off-by: Mark Stemm <mark.stemm@gmail.com>