Remove extra warning flags that are not understood by old GCC versions.
https://github.com/weechat/weechat/issues/891#issuecomment-1693308171
--- CMakeLists.txt.orig	2025-08-16 12:49:19
+++ CMakeLists.txt	2025-09-16 16:16:35
@@ -35,8 +35,6 @@
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fsigned-char -fms-extensions -Wall -Wextra")
 if(CMAKE_C_COMPILER_ID STREQUAL "GNU")
   # extra options specific to GCC
-  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wformat-overflow=2 -Wformat-truncation=2")
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wformat-overflow=2 -Wformat-truncation=2")
 endif()
 
 # version