Fix pkg-config support (closes issue #8).

Append CPPFLAGS/CFLAGS/LDADD using :=, so that _ln and _id variables
are expanded inside the .for/.endfor loop, not later on in
mkc_imp.conf-final.mk. Otherwise, only the last set of flags is added,
but multiple times (see Debian bug #741837).

Also, add -DHAVE_* flags according to the manual.
Merge pull request #9 from andrewshadura/andrewsh/fix-pkgconfig

Fix pkg-config support (closes issue #8).