synfig: fix build

Author: daeho-ro

Formula/s/synfig.rb

@@ -67,6 +67,13 @@ class Synfig < Formula
   def install
     ENV.cxx11
 
+    # Workaround to fix error: a template argument list is expected after
+    # a name prefixed by the template keyword [-Wmissing-template-arg-list-after-template-kw]
+    # PR ref: https://github.com/synfig/synfig/pull/3559
+    if DevelopmentTools.clang_build_version >= 1700
+      ENV.append_to_cflags "-Wno-missing-template-arg-list-after-template-kw"
+    end
+
     # missing install-sh in the tarball, and re-generate configure script
     # upstream bug report, https://github.com/synfig/synfig/issues/3398
     system "autoreconf", "--force", "--install", "--verbose"