Rename Makefile.linux64 to Makefile, because it also works on Mac

2 jobs for master in 5 minutes and 13 seconds
Status Job ID Name Coverage
  Test
failed #42782
build

00:00:49

passed #42783
doc

00:05:12

 
Name Stage Failure
failed
build Test
update-alternatives: using /usr/bin/g++ to provide /usr/bin/c++ (c++) in auto mode
Setting up build-essential (12.3) ...
Processing triggers for libc-bin (2.24-11+deb9u3) ...
$ make -C src -f Makefile.linux64
make: Makefile.linux64: No such file or directory
make: Entering directory '/builds/clean-and-itasks/clean-test-properties/src'
make: Leaving directory '/builds/clean-and-itasks/clean-test-properties/src'
make: *** No rule to make target 'Makefile.linux64'. Stop.
ERROR: Job failed: exit code 1