error: ‘for’ loop initial declarations are only allowed in C99 mode

I tried to install a R package (jsonlite), but I got the error:

error: ‘for’ loop initial declarations are only allowed in C99 mode

I filled a bug, but the dev replied that my compiler needs to use -std=gnu99 for compiling R packages. It seems that my Tumbleweed is not observing C and C++ standards (C99, C++98 or where declared C++11). How may I use -std=gnu99 and its standards?