build: Silence warning about _FORTIFY_SOURCE requiring optimization
Building with AX_CHECK_ENABLE_DEBUG([yes]), which is what happens by default for non-release builds, turns off compiler optimizations and overrides any optimization specified via CFLAGS in the build environment. This means that the nightly Flatpaks, and almost all other non-release builds, are built without any optimization. This triggers a warning about _FORTIFY_SOURCE requiring optimization, on distributions that build with _FORTIFY_SOURCE, when running g-ir-scanner to generate Goa-1.0.gir.
Since g-ir-scanner uses Python's distutils.ccompiler, the compiler flags with which Python itself was built gets tagged into the compilation of the generated code that's used to generate the GIR. If the Python installation being used was built with _FORTIFY_SOURCE, the warning is triggered.
It can be useful to turn off compiler optimizations to get a better debugging experience, but it becomes a problem if it stomps over the build environment while doing so. The person doing the builds should should get to decide between ease of debugging and other factors. After all, debugging is not the only thing that a developer does. One can use GDB reasonably well with the Autoconf default, which also happens to be what most distributions use, of "-g O2".
This wouldn't have been such a problem if AX_CHECK_ENABLE_DEBUG attached its flags before the values from the environment instead of after, because in case of multiple -O options, the last such option is the one that's effective.
Thankfully, "no" doesn't override the environment, which is what happens for release builds, and distributions generally set their own CFLAGS. Otherwise every single user-facing build would have been broken. Note that any release build without CFLAGS set in the environment would neither get debug symbols (ie., no "-g") nor any compiler optimization because AX_CHECK_ENABLE_DEBUG always suppresses the Autoconf defaults of "-g -O2".
One solution could have been to default to "info" for non-release builds and recommend the use of --enable-debug=info while building from Git, but that would not address release builds without CFLAGS.
Given that the only other thing the macro does is to define the NDEBUG pre-processor macro when debugging is set to "no", which isn't widely used in the GLib-based GNOME platform [1], it seems better to just remove it altogether.
[1] glib/gio/xdgmime is the only widely used code path where assert(3) is used. It's also used in gio/kqueue/dep-list.c, which is BSD-specific and in GTK+'s Broadway backend. All those can probably be replaced with g_assert.