This same code compiled and ran before I upgraded from wxWidgets 3.1.2 to 3.1.3, and I did not get these warnings.
No. Your original error was that the project used a non-existent compiler / toolchain and failed to execute a missing binary (the compiler). So you did more than "just" upgrade wxWidgets, the previous code also could not be compiled by a missing compiler. Then you did a lot of random changes that did not solve the original error and now you have a huge mess.
Upgrading wxWidgets is quite simple, especially if it's just a patchlevel upgrade, point to the new root directory (if you not simply replaced the contents of the current one) and do a FULL rebuild. That last step is the most "difficult" one, at least in my setup (CodeBlocks + MSVC) a simple Clean + Build / Rebuild in CodeBlocks does NOT remove everything so i manually delete all outputs (which is quite easy for me, because i do an out-of-tree build, i just delete the output folder, all outputs are inside there). Not sure if it works flawless with your setup, but a good candidate to get missed by the Clean step are precompiled headers.
Your warnings don't look like a x86/x64 mismatch, the linker should detect the architecture and spit out a different error / warning message (IIRC gcc does that on Linux). It looks like a static / dynamic mismatch of wxWidgets, you link the static libraries but the project is setup for dynamic libraries or reversed.
Best thing you can do is a fresh checkout of the last known working revision of your project and restart from there. It looks like your orginal project already is quite messy and contains dead settings and i don't know what else, but only upgrading wxWidgets should be an easy task. You should start with that first. And then you can try to switch the compiler and/or architecture, but for that you should better review the current settings and fix them properly.