This is the stuff to be looked at:
configure:7667: checking whether objc really works
configure:7687: gcc -o conftest -g -O2 -I/usr/local/include -I/usr/local/include -I/usr/local/include -fnext-runtime -DNeXT_RUNTIME -x objective-c -shared-libgcc -L/usr/local/lib -L/usr/local/lib -L/usr/local/lib conftest.c -ldl -lpthread -shared-libgcc -pthread -fexceptions -fobjc-nonfragile-abi -fnext-runtime -L/usr/local/lib/ -F/Library/Frameworks/ -lobjc -lm >&5
In file included from conftest.c:104:
In file included from ././config/config.objc.m:2:
././config/objc-common.g:53:3: error: assignment to Objective-C's isa is deprecated in favor of object_setClass() [-Werror,-Wdeprecated-objc-isa-usage]
obj->isa = self;
^ ~~~~~~~
object_setClass( , )
././config/objc-common.g:46:5: note: instance variable is declared here
id isa;
^
1 error generated.
Presumably base/config/objc-common.g needs to be fixed, specifically "Provide dummy implementations for NSObject and NSConstantString for libobjc2 which needs them.".
Solely based on the commend, I'd suggest surrounding that section by #if !defined(NeXT_RUNTIME)/#endif. Can you try if that works? If it does we can apply that fix.
Although, reading further, there is a special section dedicated to NeXT_RUNTIME, so that might not be it. How about, #if defined(NeXT_RUNTIME) objc_setClass(....); #else obj->isa = self; #endif?
Can someone who knows what's going on shed some light?