diff --git a/build/build-clang/build-clang.py b/build/build-clang/build-clang.py
index 853c70a78b12..f4260555ec4d 100755
--- a/build/build-clang/build-clang.py
+++ b/build/build-clang/build-clang.py
@@ -599,6 +599,7 @@ if __name__ == "__main__":
     parser.add_argument(
         "-c",
         "--config",
+        action="append",
         required=True,
         type=argparse.FileType("r"),
         help="Clang configuration file",
@@ -650,11 +651,35 @@ if __name__ == "__main__":
         cc_name = "clang-cl"
         cxx_name = "clang-cl"
 
-    config_dir = os.path.dirname(args.config.name)
-    config = json.load(args.config)
-    patches = config.get("patches")
-    if patches:
-        config["patches"] = [os.path.join(this_config_dir, p) for p in patches]
+    config = {}
+    # Merge all the configs we got from the command line.
+    for c in args.config:
+        this_config_dir = os.path.dirname(c.name)
+        this_config = json.load(c)
+        patches = this_config.get("patches")
+        if patches:
+            this_config["patches"] = [os.path.join(this_config_dir, p) for p in patches]
+        for key, value in this_config.items():
+            old_value = config.get(key)
+            if old_value is None:
+                config[key] = value
+            elif value is None:
+                if key in config:
+                    del config[key]
+            elif type(old_value) != type(value):
+                raise Exception(
+                    "{} is overriding `{}` with a value of the wrong type".format(
+                        c.name, key
+                    )
+                )
+            elif isinstance(old_value, list):
+                for v in value:
+                    if v not in old_value:
+                        old_value.append(v)
+            elif isinstance(old_value, dict):
+                raise Exception("{} is setting `{}` to a dict?".format(c.name, key))
+            else:
+                config[key] = value
 
     stages = 2
     if "stages" in config:
diff --git a/taskcluster/scripts/misc/build-clang.sh b/taskcluster/scripts/misc/build-clang.sh
index e9add71ecf02..378727c9cbe7 100755
--- a/taskcluster/scripts/misc/build-clang.sh
+++ b/taskcluster/scripts/misc/build-clang.sh
@@ -4,7 +4,7 @@ set -x -e -v
 # This script is for building clang.
 
 ORIGPWD="$PWD"
-JSON_CONFIG="$1"
+CONFIGS=$(for c; do echo -n " -c $GECKO_PATH/$c"; done)
 
 cd $GECKO_PATH
 
@@ -16,7 +16,7 @@ if [ -d "$MOZ_FETCHES_DIR/binutils/bin" ]; then
   export PATH="$MOZ_FETCHES_DIR/binutils/bin:$PATH"
 fi
 
-case "$JSON_CONFIG" in
+case "$CONFIGS" in
 *macosx64*)
   # these variables are used in build-clang.py
   export CROSS_CCTOOLS_PATH=$MOZ_FETCHES_DIR/cctools
@@ -38,7 +38,7 @@ case "$JSON_CONFIG" in
 *linux64*|*android*)
   ;;
 *)
-  echo Cannot figure out build configuration for $JSON_CONFIG
+  echo Cannot figure out build configuration for $CONFIGS
   exit 1
   ;;
 esac
@@ -47,7 +47,7 @@ esac
 set +x
 
 cd $MOZ_FETCHES_DIR/llvm-project
-python3 $GECKO_PATH/build/build-clang/build-clang.py -c $GECKO_PATH/$1
+python3 $GECKO_PATH/build/build-clang/build-clang.py $CONFIGS
 
 set -x