Add clang-tidy target to CMakeLists.txt.
Add clang-tidy target to CMakeLists.txt.

This is a "linter" that checks code for errors, taken from PR #222 by
riszotto.

dbaarda/librsync,sourcefrog/librsync,dbaarda/librsync,dbaarda/librsync,librsync/librsync,librsync/librsync,dbaarda/librsync,librsync/librsync,sourcefrog/librsync,sourcefrog/librsync,sourcefrog/librsync,librsync/librsync