Fix compiler warning on msys2:
enchmorc.c:35:31: error: unknown escape sequence: '\o'
The absolute path returned by current_source_dir()
returns backward slashes, which don't work well when
used as a C string constant. join_paths() will make
it all forward-slashes.
-bench10_orc_path = meson.current_source_dir() + '/bench10.orc'
+bench10_orc_path = join_paths(meson.current_source_dir(), 'bench10.orc')
executable ('benchmorc', 'benchmorc.c',
c_args : ['-DORC_TEST_FILENAME="' + bench10_orc_path + '"'],