Fix compilation in Xcode 15

[moffatman]

Fix #25

• Use a different approach for compiling towards simulator arm64, with the -target cflag
  • This results in dylib with the proper PLATFORM written by the linker
  • This was the issue in #25, where intermediate link stages would fail as PLATFORM didn't match
  • We can now remove the post-processing step to rewrite PLATFORM on each dylib
  • Need to patch ltmain.sh in a few recipes to pass through -target to the linker
• A few various patches to build flags to fix compilation errors

[birros]

I don't understand how the CI could succeed in the past with find_program('pkgconfig') while the binary is called pkg-config 🤔