Use path join operator for includedir and libdir in cmake config

When includedir or libdir are set to an absolute path, the previous code
appends that absolute path to ${prefix}, which results in a wrong path. The
right behavior would be in this case to drop ${prefix} and just use the
absolute path.

This is an issue in nixpkgs, where includedir is set to the final (absolute)
path of the built library in the Nix store.

See NixOS/nixpkgs#180054.

src/meson.build

@@ -657,8 +657,8 @@ endif
 have_gobject = conf.get('HAVE_GOBJECT', 0) == 1
 
 cmake_config = configuration_data()
-cmake_config.set('libdir', '${prefix}/@0@'.format(get_option('libdir')))
-cmake_config.set('includedir', '${prefix}/@0@'.format(get_option('includedir')))
+cmake_config.set('libdir', '${prefix}' / '@0@'.format(get_option('libdir')))
+cmake_config.set('includedir', '${prefix}' / '@0@'.format(get_option('includedir')))
 cmake_config.set('HB_LIBTOOL_VERSION_INFO', hb_libtool_version_info)
 cmake_config.set('have_gobject', '@0@'.format(have_gobject))
 configure_file(input: 'harfbuzz-config.cmake.in',