This reverts commit a495e6a44c.
This break extension builds:
```
/Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:321:in 'String#replace': can't modify frozen String: "$(SDKROOT)$(prefix)/include" (FrozenError)
from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:321:in 'RbConfig.expand'
from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:314:in 'block in RbConfig.expand'
from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:307:in 'String#gsub'
from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:307:in 'RbConfig.expand'
from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:314:in 'block in RbConfig.expand'
from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:307:in 'String#gsub'
from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:307:in 'RbConfig.expand'
from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:314:in 'block in RbConfig.expand'
from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:307:in 'String#gsub'
from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:307:in 'RbConfig.expand'
from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:325:in 'block in <module:RbConfig>'
from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:324:in 'Hash#each_value'
from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:324:in '<module:RbConfig>'
from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:11:in '<top (required)>'
from ./ext/extmk.rb:42:in 'Kernel#require'
from ./ext/extmk.rb:42:in '<main>'
make[1]: *** [ext/configure-ext.mk:70: ext/json/exts.mk] Error 1
```
7dbd9c2636