Fix making a packge using homebrew LLVM

Previously if using the homebrew LLVM the correct paths were not found
for openMP and the std C++ library.

MantidPlot/make_package.rb.in

@@ -108,6 +108,10 @@ if(poco_version[0] > 1 || (poco_version[0] == 1 && poco_version[1] >= 6))
 end
 
 if("@OPENMP_FOUND@" == "TRUE")
+  if not File.file?("/usr/lib/libomp.dylib")
+      # assume we're using system llvm
+      library_filenames << "libc++.dylib"
+  end
   library_filenames << "libomp.dylib"
 end
 
@@ -119,6 +123,14 @@ library_filenames.each do |filename|
     copyFile(openssl_dir+filename)
   elsif  filename.include? "libcrypto.dylib"
     copyFile(openssl_dir+filename)
+  elsif filename.include? "libomp.dylib" or filename.include? "libc++.dylib"
+      if File.file?(lib_dir+"libomp.dylib")
+        # using system llvm
+        copyFile(lib_dir+filename)
+      elsif
+        # using homebrew llvm
+        copyFile("/usr/local/opt/llvm/lib/"+filename)
+      end
   else
     copyFile(lib_dir+filename)
   end