Buildsystem: Refactor compiler detection code

* Delete old check for gcc 8 as we support 9 or higher
* Flatten branches for clang and apple clang
* Renamed is_vanilla_clang to is_apple_clang to be more clear

Signed-off-by: Yevhen Babiichuk (DustDFG) <dfgdust@gmail.com>

methods.py

@@ -652,7 +652,9 @@ def detect_darwin_sdk_path(platform, env):
             raise
 
 
-def is_vanilla_clang(env):
+def is_apple_clang(env):
+    if env["platform"] not in ["macos", "ios"]:
+        return False
     if not using_clang(env):
         return False
     try:
@@ -660,7 +662,7 @@ def is_vanilla_clang(env):
     except (subprocess.CalledProcessError, OSError):
         print_warning("Couldn't parse CXX environment variable to infer compiler version.")
         return False
-    return not version.startswith("Apple")
+    return version.startswith("Apple")
 
 
 def get_compiler_version(env):