configure: Reworked common cflags and lib variables for incubator.

Introduced INCUBATOR_LIBS and INCUBATOR_CFLAGS make variables to be able to
link modules more easily, and added convenient INCUBATOR_TEST_LDADD
variable for unit test linking.
Merge pull request #86 from talien/f/make-variables-rework

configure: Reworked common cflags and lib variables for incubator.