ci: Explicitly specify C++ compilers

This may be necessary in certain situations, for example when performing
static analysis with scan-build, to ensure that the standard C++ library
is added to the linker. See
panel-plugins/xfce4-sensors-plugin#48 (comment 90769)

ci/build_project.yml

@@ -74,6 +74,7 @@ translations:
 build-gcc:
   variables:
     CC: "gcc"
+    CXX: "g++"
     CONFIGURE_FLAGS: "--enable-debug=werror"
   before_script:
     # We can't use shell parameter expansion under the variables keyword above,
@@ -87,9 +88,10 @@ build-gcc:
 build-clang:
   variables:
     CC: "clang"
+    CXX: "clang++"
     CONFIGURE_FLAGS: "--enable-debug=werror"
   before_script:
-    - WRAPPER=${SCAN_BUILD+scan-build --use-cc=clang --use-c++=clang}
+    - WRAPPER=${SCAN_BUILD+scan-build --use-cc=clang --use-c++=clang++}
   extends: .build
 
 build-no-x11: