Eliminate gyp/build output

[jfirebaugh]

Some build products get output to gyp/build rather than build. This is probably a historical accident or quirk of gyp. We should adjust the build to eliminate this special case output directory -- everything should go to build.

[jfirebaugh]

gyp is involved here, but this largely seems to be a quirk of xcodebuild -target foo versus xcodebuild -scheme foo. The former seems to set SYMROOT to $SRCROOT/build (and SRCROOT is indeed set to ./gyp by gyp), while the latter uses a directory in ~/Library/Developer/Xcode/DerivedData (the same one that builds from within Xcode use). I can't find this difference between -target and -scheme documented anywhere, and it's contrary to the output of xcodebuild -showBuildSettings, which says that SYMROOT is the ~/Library/... path in either case.

[mikemorris]

IIRC this was a result of some refactoring I did on the Node.js bindings when we integrated them into this repository.

[jfirebaugh]

The key here seems to be that SYMROOT needs to be an absolute path. If it's a relative path, it's ignored by xcodebuild -target ..., but if it's absolute, it's respected.

In #4608 I'm setting up the ios and osx gyp files to force an absolute path.