tests: scripts: fix test_cmake_out_of_source

By default C++ code would be compiled with GNU while C with Clang and
this can create problems at link time. In order to prevent this we
use Clang for both.

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/tests/scripts/components-build-system.sh b/tests/scripts/components-build-system.sh
index 3108aa7..e533cdf 100644
--- a/tests/scripts/components-build-system.sh
+++ b/tests/scripts/components-build-system.sh
@@ -65,7 +65,9 @@
     mkdir "$OUT_OF_SOURCE_DIR"
     cd "$OUT_OF_SOURCE_DIR"
     # Note: Explicitly generate files as these are turned off in releases
-    cmake -D CMAKE_BUILD_TYPE:String=Check -D GEN_FILES=ON -D TEST_CPP=1 "$MBEDTLS_ROOT_DIR"
+    # Note: Use Clang compiler also for C++ (C uses it by default)
+    CXX=clang++ cmake -D CMAKE_BUILD_TYPE:String=Check -D GEN_FILES=ON \
+                      -D TEST_CPP=1 "$MBEDTLS_ROOT_DIR"
     make
 
     msg "test: cmake 'out-of-source' build"