Mojave vs. GCC

After you've installed Xcode (or, my preference, the Command Line Tools via xcode-select -install) so that you can install and use GNAT, you may expect to be able to compile C code too.

Mojave may surprise you with

$ gcc casing.c -o casing
casing.c:1:10: fatal error: stdio.h: No such file or directory
    1 | #include <stdio.h>
      |          ^~~~~~~~~
      compilation terminated.

The reason, according to this question and its answers, is that Apple's developer tools, in particular the clang compiler, know where to find the include files under /Library/Developer; GCC doesn't (I'm sure it could be made to, but ...) and so we have to add an extra step to install them in the normal place:

$ sudo installer -pkg /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg -target /
Password:
installer: Package name is macOS_SDK_headers_for_macOS_10.14
installer: Installing at base path /
installer: The install was successful.

You may need to repeat this after macOS or Command Line Tools (or Xcode) updates.

Mojave vs. GDB

Apple's software development tools are based on LLVM, and Apple don't seem to feel it necessary to keep GCC and friends up to date with changes in the Apple tools or security policies.

GDB has been particularly affected by this. You can see why a tool which is capable of interacting with running programs would have to be treated with caution.