Handle spaces in parent directory name

[jhayes-dev]

If there is a space in the parent directory name farmhash will not compile. If fails with:

CXX(target) Release/obj.target/farmhash/src/upstream/farmhash.o
clang: error: no such file or directory: 'space/node_modules/node-addon-api'
make: *** [Release/obj.target/farmhash/src/upstream/farmhash.o] Error 1
gyp ERR! build error
gyp ERR! stack Error: make failed with exit code: 2
gyp ERR! stack at ChildProcess.onExit (ROOT/src/framfash-with space/node_modules/node-gyp/lib/build.js:194:23)
gyp ERR! stack at ChildProcess.emit (node:events:513:28)
gyp ERR! stack at Process.ChildProcess._handle.onexit (node:internal/child_process:293:12)
gyp ERR! System Darwin 21.5.0
gyp ERR! command "ROOT/.nvm/versions/node/v16.18.0/bin/node" "ROOT/src/framfash-with space/node_modules/.bin/node-gyp" "rebuild"
gyp ERR! cwd ROOT/src/framfash-with space
gyp ERR! node -v v16.18.0
gyp ERR! node-gyp -v v6.1.0

The generated build/farmhash.target.mk does not quote the include path.

This change uses a relative path which is handled correctly.

[lovell]

I've left a comment inline as there's one small change required to use include_dir in a scalar rather than array context, otherwise this is good to go.