Missing -soname support

[rurban]

eg
chibicc -shared -DPIC .libs/dwg.o .libs/common.o .libs/bits.o .libs/decode.o .libs/decode_r11.o .libs/decode_r2007.o .libs/reedsolomon.o .libs/print.o .libs/free.o .libs/hash.o .libs/dynapi.o .libs/classes.o .libs/dwg_api.o .libs/objects.o .libs/out_dxf.o .libs/out_dxfb.o .libs/out_json.o .libs/out_geojson.o .libs/encode.o .libs/dxfclasses.o .libs/in_dxf.o .libs/in_json.o -lm -lc -g -soname libredwg.so.0 -o .libs/libredwg.so.0.0.13
a normal autotools project.

I'll try to fix it