modify code to allow shared object used independently and some minor changes

[hwyao]

modify code to allow shared object used independently and some minor changes：

• update the target ${PROJECT_NAME}-shared with correct shared object linking dependency on osqp and qpOASES.
• Correspondingly, update the way of linking in example and tests.
• Switch gcc argument style linking to target-based linking with add_library(IMPORTED)
• remove add_dependencies() of a target over a ExternalProject, which actually does nothing.