configry: use $(JANSSON_CPPFLAGS) instead of explicit flags

Set JANSSON_CPPFLAGS in configure.ac and use $(JANSSON_CPPFLAGS) in
all the relevant Makefile.am's.  This allows us a little more central
control of what CPPFLAGS Jansson needs (and is also a half-step in the
right direction of supporting compiling against an external Jansson
someday).