impossible to redefine GCC_ENABLE_CPP_RTTI in an addon's common.gypi

• Node Version: At least since Node 12.x
• Platform: All OSX versions
• Compiler: all XCode versions
• Module: node-gdal-async

The way Node's common.gypi is included, by using -I and at the end of the list, makes it impossible for an addon to override the xcode_settings in its own common.gypi. RTTI is disabled by default in Node.js. Enabling it on Linux is possible, since one has to remove the cflags_cc element:

"cflags_cc!": ["-fno-rtti", "-fno-exceptions"]

When there are both cflags_cc and cflags_cc!, the negative one takes precedence.
However on OSX, this is a setting:

"xcode_settings": {
  "GCC_ENABLE_CPP_RTTI": "YES",
  "GCC_ENABLE_CPP_EXCEPTIONS": "YES",
 }

In this case the last one to modify it, wins.
One can still override it in the gyp file itself, but it is not possible to override it in an include file.
I have 105 gyp files (lots of plugins) in node-gdal-async and I simply need to be able to specify options in the include file.

Maybe include the project's own common.gypi after the one from Node.js?

Start by tracing how Node's common.gypi is included with -I and how include ordering affects xcode_settings, using the node-gdal-async setup as the reproduction. Done means an addon's own common.gypi can override GCC_ENABLE_CPP_RTTI and related settings without editing every gyp file.