Bug 1236577 - turn off -Wignored-attributes when compiling with clang-cl; r=glandium

Several JS engine header files, notable RootingAPI.h, do:

class JS_PUBLIC_API(...) { ... };

which sticks __declspec(dllimport) on the type.  clang-cl warns us that
it doesn't implement that, but since we're compiling with clang-cl as an
interesting side project, rather than shipping releases with it, we can
ignore that warning.

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"

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"