build not idempotent

[umlaeute]

running the build multiple times, results in an error:

$ make clean
++++ info: using Makefile.pdlibbuilder version 0.4.4
++++ info: making target clean in lib purest_json
rm -f src/json-decode.o src/json-encode.o src/oauth.o src/rest.o src/urlparams.o   src/purest_json.o
rm -f json-encode.pd_linux rest.pd_linux urlparams.pd_linux oauth.pd_linux json-decode.pd_linux purest_json.pd_linux 
rm -f *.pre *.lst

$ make
++++ info: using Makefile.pdlibbuilder version 0.4.4
++++ info: using Pd API /usr/include/pd/m_pd.h
++++ info: making target all in lib purest_json
++++ info: making src/json-encode.o in lib purest_json
[...]
++++ info: linking objects in json-decode.pd_linux for lib purest_json
cc -rdynamic -shared -fPIC -Wl,-rpath,"\$ORIGIN",--enable-new-dtags    -o json-decode.pd_linux src/json-decode.o  -lc -lm -lcurl -ljson-c -loauth  
++++info: target all in lib purest_json completed

$ make
++++ info: using Makefile.pdlibbuilder version 0.4.4
++++ info: using Pd API /usr/include/pd/m_pd.h
++++ info: making target all in lib purest_json
++++ info: evaluating implicit prerequisites in lib purest_json.....
cc: error: src/json-decode: Datei oder Verzeichnis nicht gefunden
cc: fatal error: no input files
compilation terminated.
pd-lib-builder/Makefile.pdlibbuilder:921: *** missing separator.  Stop.

@katjavetter this is probably an issue with pd-lib-builder, rather than PuRestJson.

i've also tested with pd-lib-builder 0.5.0 (using #45), with the same result:

$ make PDLIBBUILDER_DIR=/usr/share/pd-lib-builder
++++ info: using Makefile.pdlibbuilder version 0.5.0
++++ info: using Pd API /usr/include/pd/m_pd.h
++++ info: making target all in lib purest_json
++++ info: evaluating implicit prerequisites in lib purest_json.....
cc: error: src/json-decode: No such file or directory
cc: fatal error: no input files
compilation terminated.
/usr/share/pd-lib-builder/Makefile.pdlibbuilder:959: *** missing separator.  Stop.