Upgrade to C++17 [CMake] (#4717)

* Make our CMake slightly less insane

CMake/Fixups.cmake

@@ -0,0 +1,14 @@
+# TODO these should be in the submodules
+# Under Windows, we need Lua as DLL; on *nix we need it linked statically:
+if (WIN32)
+	target_compile_definitions(lua PUBLIC LUA_BUILD_AS_DLL)
+endif()
+
+# Let Lua use additional checks on its C API. This is only compiled into Debug builds:
+target_compile_definitions(lua PRIVATE LUA_USE_APICHECK)
+
+if(NOT MSVC AND "${CMAKE_SYSTEM_PROCESSOR}" MATCHES "arm")
+	# mbed TLS uses the frame pointer's register in inline assembly for its bignum implementation:
+	# https://tls.mbed.org/kb/development/arm-thumb-error-r7-cannot-be-used-in-asm-here
+	target_compile_options(mbedcrypto PRIVATE -fomit-frame-pointer)
+endif()