Set C99 as the explicit, not implicit, build standard.

CMakeLists.txt

@@ -13,6 +13,9 @@ project(mosquitto
 	LANGUAGES C CXX
 )
 
+set(CMAKE_C_STANDARD 11)
+set(CMAKE_C_STANDARD_REQUIRED ON)
+
 list(APPEND CMAKE_MODULE_PATH "${PROJECT_SOURCE_DIR}/cmake/")
 
 add_definitions (-DCMAKE -DVERSION=\"${VERSION}\")

ChangeLog.txt

@@ -160,6 +160,7 @@ Build:
 - Increased CMake minimal required version to 3.14, which is required for the
   preinstalled SQLite3 find module.
 - Add an CMake option `WITH_LTO` to enable/disable link time optimization.
+- Set C99 as the explicit, rather than implicit, build standard.
 
 
 2.0.15 - 2022-08-16

config.mk

@@ -164,7 +164,7 @@ ifeq ($(UNAME),SunOS)
 		CFLAGS?=-Wall -ggdb -O2
 	endif
 else
-	CFLAGS?=-Wall -ggdb -O3 -Wconversion -Wextra
+	CFLAGS?=-Wall -ggdb -O3 -Wconversion -Wextra -std=gnu99
 endif
 
 STATIC_LIB_DEPS:=