Make sure we get C99

The compiler might default to something older, so make sure we get
the features we need.

CMakeLists.txt

@@ -68,6 +68,9 @@ set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "${CMAKE_CXX_FLAGS_RELWITHDEBINFO} -UNDEBUG")
 set(CMAKE_C_FLAGS_MINSIZEREL "${CMAKE_C_FLAGS_MINSIZEREL} -UNDEBUG")
 set(CMAKE_CXX_FLAGS_MINSIZEREL "${CMAKE_CXX_FLAGS_MINSIZEREL} -UNDEBUG")
 
+# Make sure we get a sane C version
+set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=gnu99")
+
 # Tell the compiler to be stringent
 set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -Wformat=2")
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wformat=2")