[tests] add build tests for BR features (#6570)

BR features are tested with "Border Router" CI task but it is built
with gcc by default. This commit adds BR features to posix build
checks which apply clang/clang++.

script/check-posix-build-cmake

@@ -51,6 +51,13 @@ build()
         "-DOT_TREL=ON"
     )
 
+    if [[ $OSTYPE != "darwin"* ]]; then
+        options+=(
+            "-DOT_BORDER_ROUTING=ON"
+            "-DOT_SRP_SERVER=ON"
+        )
+    fi
+
     reset_source
     "$(dirname "$0")"/cmake-build posix "${options[@]}"
 }

src/posix/platform/infra_if.cpp

@@ -295,7 +295,7 @@ uint32_t InfraNetif::GetFlags(void) const
 
     close(sock);
 
-    return ifReq.ifr_flags;
+    return static_cast<uint32_t>(ifReq.ifr_flags);
 }
 
 void InfraNetif::CountAddresses(otSysInfraNetIfAddressCounters &aAddressCounters) const