diff --git a/configure.in b/configure.in
index 14a7bcf0fa82..0660aaf56ecc 100644
--- a/configure.in
+++ b/configure.in
@@ -2288,6 +2288,11 @@ ia64*-hpux*)
             # At least one MSVC header and several headers in-tree have
             # unused typedefs, so turn this on.
             CXXFLAGS="$CXXFLAGS -Wno-unused-local-typedef"
+            # Several JS engine header files use __declspec(dllimport) on
+            # classes, and clang-cl helpfully warns about its non-support
+            # for such cases.  We're not particularly worried about that,
+            # so ignore that warning.
+            CXXFLAGS="$CXXFLAGS -Wno-ignored-attributes"
         fi
         # make 'foo == bar;' error out
         CFLAGS="$CFLAGS -we4553"
diff --git a/js/src/configure.in b/js/src/configure.in
index a1495970282c..3b65e6a555ea 100644
--- a/js/src/configure.in
+++ b/js/src/configure.in
@@ -1768,6 +1768,11 @@ ia64*-hpux*)
             # At least one MSVC header and several headers in-tree have
             # unused typedefs, so turn this on.
             CXXFLAGS="$CXXFLAGS -Wno-unused-local-typedef"
+            # Several JS engine header files use __declspec(dllimport) on
+            # classes, and clang-cl helpfully warns about its non-support
+            # for such cases.  We're not particularly worried about that,
+            # so ignore that warning.
+            CXXFLAGS="$CXXFLAGS -Wno-ignored-attributes"
         fi
         # make 'foo == bar;' error out
         CFLAGS="$CFLAGS -we4553"