test: make the c++ build test mirror the c build test

Part-of: <https://gitlab.freedesktop.org/libinput/libei/-/merge_requests/310>

test/buildtest.cc

@@ -1,5 +1,10 @@
+#if INCLUDE_LIBEI
 #include <libei.h>
+#endif
+
+#if INCLUDE_LIBEIS
 #include <libeis.h>
+#endif
 
 /* This is a build-test only */
 

test/meson.build

@@ -112,10 +112,17 @@ executable('test-build-libeis',
     install : false)
 
 if add_languages('cpp', required: false)
-    executable('test-build-cxx',
+    executable('test-build-cxx-libei',
             'buildtest.cc',
-            dependencies: [dep_libei, dep_libeis],
+            dependencies: [dep_libei],
             include_directories: [inc_src],
+            c_args : ['-Werror', '-DINCLUDE_LIBEI=1'],
+            install: false)
+    executable('test-build-cxx-libeis',
+            'buildtest.cc',
+            dependencies: [dep_libeis],
+            include_directories: [inc_src],
+            c_args : ['-Werror', '-DINCLUDE_LIBEIS=1'],
             install: false)
 endif