Use -Wno-unused-but-set-variable only if supported

This fixes the following clang's warning

warning: unknown warning option '-Wno-unused-but-set-variable'; did you mean
      '-Wno-unused-const-variable'? [-Wunknown-warning-option]

CMakeLists.txt

@@ -310,7 +310,10 @@ macro(ADD_C_COMPILER_FLAG_IF_AVAILABLE flag have)
 endmacro(ADD_C_COMPILER_FLAG_IF_AVAILABLE)
 
 if(CMAKE_CXX_COMPILER_ID MATCHES "GNU" OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wno-comment -Wno-reorder -Wno-unused-but-set-variable -Wno-unused-variable -Wtype-limits -std=c++11")
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wno-comment -Wno-reorder -Wno-unused-variable -Wtype-limits -std=c++11")
+
+  ADD_C_COMPILER_FLAG_IF_AVAILABLE("-Wno-unused-but-set-variable" HAVE_WNO_UNUSED_BUT_SET_VARIABLE)
+  ADD_CXX_COMPILER_FLAG_IF_AVAILABLE("-Wno-unused-but-set-variable" HAVE_WNO_UNUSED_BUT_SET_VARIABLE)
 
   find_package(SSE)
   if (HAVE_AVX2)
@@ -329,6 +332,7 @@ if(CMAKE_CXX_COMPILER_ID MATCHES "GNU" OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
 endif(CMAKE_CXX_COMPILER_ID MATCHES "GNU" OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
 
 ADD_C_COMPILER_FLAG_IF_AVAILABLE("-Werror=incompatible-pointer-types" HAVE_ERROR_INCOMPATIBLE)
+ADD_CXX_COMPILER_FLAG_IF_AVAILABLE("-Werror=incompatible-pointer-types" HAVE_ERROR_INCOMPATIBLE)
 
 if(CMAKE_C_COMPILER_ID MATCHES "GNU" OR CMAKE_C_COMPILER_ID MATCHES "Clang")
   set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -Wno-comment -Wno-write-strings -Winline -Wno-unused-result -Wformat -Wmissing-field-initializers -Wtype-limits -std=c99 -D_GNU_SOURCE")

lib/src/asn1/CMakeLists.txt

@@ -18,7 +18,7 @@
 # and at http://www.gnu.org/licenses/.
 #
 
-set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-switch -Wno-unused-but-set-variable -Wno-unused-variable -Wno-return-type -Wno-sign-compare -Wno-reorder -Wno-parantheses -fno-exceptions")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-switch -Wno-unused-variable -Wno-return-type -Wno-sign-compare -Wno-reorder -Wno-parantheses -fno-exceptions")
 add_library(srslte_asn1 STATIC
   liblte_common.cc
   liblte_mme.cc