build: call AM_PROG_CC_C_O as requested by autoreconf
config.h.in: delete, this is a generated file
build: remove non-existent directory from EXTRA_DIST
build: link libjson.la against libjson-c.la

By using libtool linkage the dependencies are automatically created, so parallel
builds just work.
tests: pass top_srcdir so the tests compile in distcheck
Ignore compile and config.h.in