drop -std=c89, fails with clang 13

cf https://github.com/MapServer/mapcache/pull/290#issuecomment-1410089296

Index: CMakeLists.txt
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -82,7 +82,7 @@ if(CMAKE_COMPILER_IS_GNUCC)
 endif()
 
 if(CMAKE_C_COMPILER_ID MATCHES "Clang")
-  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -Werror=declaration-after-statement -std=c89 -Wno-comment")
+  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -Werror=declaration-after-statement -Wno-comment")
 endif()
 
 #options suported by the cmake builder
