Make function calls without prior declaration an error

which is the default behavior on C99 and up.

CMakeLists.txt

@@ -12,10 +12,16 @@ else()
   set (CMAKE_C_STANDARD 99)
 endif()
 include(CheckCCompilerFlag)
-CHECK_C_COMPILER_FLAG("-Werror=pointer-arith" COMPILER_HAS_POINTER_ARITH_TOGGLE)
-if(COMPILER_HAS_POINTER_ARITH_TOGGLE)
-    set(CMAKE_C_FLAGS "-Werror=pointer-arith ${CMAKE_C_FLAGS}")
-endif()
+foreach(option -Werror=pointer-arith;-Werror=implicit-function-declaration)
+    CHECK_C_COMPILER_FLAG("${option}" COMPILER_HAS_THOSE_TOGGLES)
+    set(outcome "Failed")
+    if(COMPILER_HAS_THOSE_TOGGLES)
+        set(CMAKE_C_FLAGS "${option} ${CMAKE_C_FLAGS}")
+        set(outcome "works")
+    endif()
+    message(STATUS "Testing if ${option} can be used -- ${outcome}")
+endforeach()
+
 
 add_subdirectory(src release)
 

src/Makefile

@@ -262,7 +262,8 @@ endif
 #  -Wno-missing-braces   ignore invalid warning (GCC bug 53119)
 #  -D_DEFAULT_SOURCE     use with -std=c99 on Linux and PLATFORM_WEB, required for timespec
 #  -Werror=pointer-arith catch unportable code that does direct arithmetic on void pointers
-CFLAGS += -O1 -Wall -std=c99 -D_DEFAULT_SOURCE -fgnu89-inline -Wno-missing-braces -Werror=pointer-arith
+#  -Werror=implicit-function-declaration catch function calls without prior declaration
+CFLAGS += -O1 -Wall -std=c99 -D_DEFAULT_SOURCE -fgnu89-inline -Wno-missing-braces -Werror=pointer-arith -Werror=implicit-function-declaration
 
 ifeq ($(RAYLIB_BUILD_MODE), DEBUG)
     CFLAGS += -g