Set C standard to GNU C11

Versions of gcc prior to 5.1.0 need this as -std=gnu90 is the implied standard
for those, which doesn't let the project compile.

linux/Makefile

@@ -10,7 +10,7 @@ SRC_PATH = src
 # Space-separated pkg-config libraries used by this project
 LIBS =
 # General compiler flags
-COMPILE_FLAGS = -Wall -Wextra -fstrict-aliasing
+COMPILE_FLAGS = -Wall -Wextra -fstrict-aliasing -std=gnu11
 # Additional release-specific flags
 RCOMPILE_FLAGS = -D NDEBUG -O2 
 # Additional debug-specific flags