static sdai libs need include paths
don't set output dirs if SC_IS_SUBBUILD
if shared libs are not built, don't append -static to static lib names
install generated sdai headers
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
copy to the object in ptr if it exists, else overwrite ptr contents with object pointer

Tried to delete contents of ptr if non-null, but it frequently points to a static data member - in which case
delete fails noisily.
So, check if null. if not null, use set_null() (if it exists) and operator=; otherwise, overwrite the
pointer

Also make SDAI_Enum dtor public
Merge pull request #275 from stepcode/review/subbuild

Improve application building experience when using SC as a subbuild