build: remove gyp build files (#14097)

* build: remove gyp build files

* docs: update build instructions

* build: temporary restore electron.gyp

* build: do not update Electron version in the electron.gyp

* chore: remove unused submodules

* ci: remove obsolete CI scripts and configs

* chore: remove obsolete scripts

* chore: remove obsolete functions from lib/util.py

* ci: send Slack notification for nightly build results

docs/development/build-instructions-macos.md

@@ -0,0 +1,62 @@
+# Build Instructions (macOS)
+
+Follow the guidelines below for building Electron on macOS.
+
+## Prerequisites
+
+* macOS >= 10.11.6
+* [Xcode](https://developer.apple.com/technologies/tools/) >= 8.2.1
+* [node.js](https://nodejs.org) (external)
+* Python 2.7 with support for TLS 1.2
+
+## Python
+
+Please also ensure that your system and Python version support at least TLS 1.2.
+This depends on both your version of macOS and Python. For a quick test, run:
+
+```sh
+$ npm run check-tls
+```
+
+If the script returns that your configuration is using an outdated security
+protocol, you can either update macOS to High Sierra or install a new version
+of Python 2.7.x. To upgrade Python, use [Homebrew](https://brew.sh/):
+
+```sh
+$ brew install python@2 && brew link python@2 --force
+```
+
+If you are using Python as provided by Homebrew, you also need to install
+the following Python modules:
+
+* [pyobjc](https://pythonhosted.org/pyobjc/install.html)
+
+## macOS SDK
+
+If you're developing Electron and don't plan to redistribute your
+custom Electron build, you may skip this section.
+
+For certain features (e.g. pinch-zoom) to work properly, you must target the
+macOS 10.10 SDK.
+
+Official Electron builds are built with [Xcode 8.2.1](http://adcdownload.apple.com/Developer_Tools/Xcode_8.2.1/Xcode_8.2.1.xip), which does not contain
+the 10.10 SDK by default. To obtain it, first download and mount the
+[Xcode 6.4](http://developer.apple.com/devcenter/download.action?path=/Developer_Tools/Xcode_6.4/Xcode_6.4.dmg)
+DMG.
+
+Then, assuming that the Xcode 6.4 DMG has been mounted at `/Volumes/Xcode` and
+that your Xcode 8.2.1 install is at `/Applications/Xcode.app`, run:
+
+```sh
+cp -r /Volumes/Xcode/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/
+```
+
+You will also need to enable Xcode to build against the 10.10 SDK:
+
+- Open `/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Info.plist`
+- Set the `MinimumSDKVersion` to `10.10`
+- Save the file
+
+## Building Electron
+
+See [Build Instructions: GN](build-instructions-gn.md).