Okay you definitely need a different toolchain.
Do a File → Settings → Build, Execution and Deployment → Toolchain, click on the Add / plus button, and select MinGW.
Now sadly one can not select the the toolchain root folder C:\Users\<user>\.platformio\packages\toolchain-atmelavr\ for “environment” (you said you’re working with an AVR based Arduino Uno so the right toolchain in this case is toolchain-atmelavr, might be different for diffrerent people). So please download MinGW from http://mingw-w64.org/doku.php/download/mingw-builds and install it, if you don’t already have one. Then your “environment” should be something like “C:\Program Files (x86)\mingw-w64\i686-7.2.0-posix-dwarf-rt_v5-rev1\mingw32”.
For the C, C++ compiler and Debugger you should however then select the avr-gcc, avr-g++ and avr-gdb files from C:\Users\<user>\.platformio\packages\toolchain-atmelavr\bin.
Works for me.
