drivers/mq3: add extern "C" to header
Merge pull request #1918 from LudwigOrtmann/1703-fixup

drivers/mq3: add extern "C" to header
cpu/stm32f0: optimized GPIO driver implementation
Merge pull request #1865 from haukepetersen/fix_stm32f0_gpioremodel

cpu/stm32f0: optimized GPIO driver implementation