Bug 1448978 - use more aggressive optimization flags for clang-cl; r=chmanchester

We want to use clang-cl to ship Firefox, and the only way we can come
close to MSVC numbers is to use -O2 here, for better or worse.

old-configure.in

@@ -1016,7 +1016,13 @@ case "$target" in
         LIBS="$LIBS kernel32.lib user32.lib gdi32.lib winmm.lib wsock32.lib advapi32.lib secur32.lib"
         MOZ_DEBUG_LDFLAGS='-DEBUG -DEBUGTYPE:CV'
         WARNINGS_AS_ERRORS='-WX'
-        MOZ_OPTIMIZE_FLAGS='-O1 -Oi'
+        # Use a higher optimization level for clang-cl, so we can come closer
+        # to MSVC's performance numbers (see bug 1443590).
+        if test -n "$CLANG_CL"; then
+            MOZ_OPTIMIZE_FLAGS='-O2'
+        else
+            MOZ_OPTIMIZE_FLAGS='-O1 -Oi'
+        fi
         MOZ_FIX_LINK_PATHS=
         LDFLAGS="$LDFLAGS -LARGEADDRESSAWARE -NXCOMPAT"
         if test -z "$DEVELOPER_OPTIONS"; then