CMakeLists.txt: Apply CXX deprecated flags conditionally

In ef941e5, we replaced the `-Wdeprecated-declarations`
flag by `-Wdepracted` and` -Wno-error=deprecated`.
Unfortunately, GCC and Clang interpret `-Wdeprecated`
differently, compare:
* https://gcc.gnu.org/onlinedocs/gcc-13.2.0/gcc/Warning-Options.html#index-Wno-deprecated
* https://gcc.gnu.org/onlinedocs/gcc-13.2.0/gcc/Deprecated-Features.html
and
* https://clang.llvm.org/docs/DiagnosticsReference.html#wdeprecated
As a result, `-Wno-error=deprecated` doesn't prevent
GCC from turning warnings related to using functions,
variables, or types marked as deprecated into errors.
Clang, however, does that.

In this commit, we conditionally enable the intended
options depending on which compiler is used.

CMakeLists.txt

@@ -805,7 +805,10 @@ if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
  list (APPEND Seastar_PRIVATE_CXX_FLAGS
  -Wno-error=stringop-overflow
  -Wno-error=array-bounds)
- endif()
+ endif ()
+ list (APPEND Seastar_PRIVATE_CXX_FLAGS
+ -Wdeprecated-declarations
+ -Wno-error=deprecated-declarations)
 endif ()
 
 if (NOT BUILD_SHARED_LIBS)