cmake: bump minimum required GCC version up to 8

Change-Id: I67e37155cdca75b83943497c008aaf18edb9a95f

src/libclient/CMakeLists.txt

@@ -37,11 +37,11 @@ project(jami-libclient)
 if(CMAKE_COMPILER_IS_GNUCC)
   execute_process(COMMAND ${CMAKE_C_COMPILER} -dumpversion
     OUTPUT_VARIABLE GCC_VERSION)
-  if(GCC_VERSION VERSION_GREATER_EQUAL 4.8)
-    message(STATUS "Found GCC version >= 4.8: " ${GCC_VERSION})
+  if(GCC_VERSION VERSION_GREATER_EQUAL 8.0)
+    message(STATUS "Found GCC version >= 8.0: " ${GCC_VERSION})
   else()
     message(FATAL_ERROR "Your version of GCC is too old, \
-please install GCC 4.8 or later")
+please install GCC 8.0 or newer")
   endif()
 endif()
 
@@ -108,33 +108,26 @@ endif()
 #     grep -E "\-W[a-zA-Z=-]*" -o > /tmp/49;
 #   sort /tmp/48 /tmp/49 | uniq -u
 if(CMAKE_COMPILER_IS_GNUCC)
-  if(GCC_VERSION VERSION_GREATER_EQUAL 4.9)
   add_definitions(
+    # GCC 4.9
     -Wconditionally-supported
     -Wno-cpp
     -Wdouble-promotion
     -Wdate-time
-      -Wfloat-conversion)
-  endif()
-
-  if(GCC_VERSION VERSION_GREATER_EQUAL 5.1)
-    add_definitions(
+    -Wfloat-conversion
+    # GCC 5.1
     -Wformat-signedness
     -Wnormalized
     -Wshift-count-negative
     -Wshift-count-overflow
     -Wsized-deallocation
-      -Wsizeof-array-argument)
-  endif()
-
-  if(GCC_VERSION VERSION_GREATER_EQUAL 6.0)
-    add_definitions(
+    -Wsizeof-array-argument
+    # GCC 6.0
     -Wnull-dereference
     -Wshift-negative-value
     -Wshift-overflow
     -Wduplicated-cond)
 endif()
-endif()
 
 if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
   add_definitions(