oops, install inside include/ dir
make build logic more robust - more checks, print error messages

* check whether STEPCODE_ROOT_DIR is relative or absolute
* exit with error message if that dir or the schema can't be found
oops, STEPattribute needs setters now that ptr is private
Default to a debug build
use STEPattribute getters, which weren't necessary with v0.7

In v0.7, STEPattribute::ptr was public