It seems that you also need to add these .c files to `/ta-lib/c/make/gen_make/ta_regtest/ta_regtest.pro'

And run perl /ta-lib/c/make/gen_make/gen_make.pl to regenerate all make files in c/make/c* subfolders.

The perl script will require tmake tool that could be installed with apt-get install tmake.

.pro file is Qt project file and tmake is Qt's make. For some reason author uses Qt's buildchain to generate Makefiles.