fix: only treat warnings as errors in newer gcc

The gcc versions older than 4.3 cannot build the current code base
without warnings, which are treated as errors, hence cannot build
argtable3. Besides, these old versions cannot enforce standard
compliance properly.

In this patch, we enhance the CMakeList.txt script so we
don't treat warnings as errors in very old gcc versions.

Author: tomghuang

src/CMakeLists.txt

@@ -46,9 +46,9 @@ else(CMAKE_C_COMPILER_ID MATCHES "GNU|Clang")
   set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wconversion")
   set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wextra")
   set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall")
-  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Werror")
-  if ((CMAKE_C_COMPILER_ID MATCHES "GNU" AND CMAKE_C_COMPILER_VERSION VERSION_GREATER 4.2)
+  if ((CMAKE_C_COMPILER_ID MATCHES "GNU" AND CMAKE_C_COMPILER_VERSION VERSION_GREATER 4.3)
     OR CMAKE_C_COMPILER_ID MATCHES "Clang")
+      set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Werror")
       set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wpedantic")
   endif()
 endif()