csharp: Add a target to generate NuGet package.

This commit adds the option to run `ninja nuget_pack` to create a NuGet
package containing both the EFL# dll and xml doc file.

It may help referencing EFL# in IDE's, dotnet projects, etc.

src/bindings/mono/efl_mono.dll.nuspec.in

@@ -0,0 +1,19 @@
+<?xml version="1.0"?>
+<package >
+  <metadata>
+    <id>Efl.CSharp</id>
+    <version>@EFL_VERSION@</version>
+    <authors>EFL Team</authors>
+    <owners>EFL Team</owners>
+    <license type="expression">Apache-2.0</license>
+    <projectUrl>http://www.enlightenment.org</projectUrl>
+    <requireLicenseAcceptance>false</requireLicenseAcceptance>
+    <description>EFL C# Bindings</description>
+    <copyright>Copyright 2019</copyright>
+    <tags>EFL</tags>
+  </metadata>
+  <files>
+    <file src="efl_mono.dll" target="lib/net46" />
+    <file src="efl_mono.xml" target="lib/net46" />
+  </files>
+</package>

src/bindings/mono/meson.build

@@ -135,6 +135,19 @@ efl_mono_dll_config = configure_file(input : 'efl_mono.dll.config.in',
                                  output : 'efl_mono.dll.config',
                                  configuration : efl_mono_conf_data)
 
+nuget_conf_data = configuration_data()
+
+nuget_conf_data.set('EFL_VERSION', meson.project_version())
+
+nuget_nuspec = configure_file(input : 'efl_mono.dll.nuspec.in',
+                              output : 'efl_mono.dll.nuspec',
+                              configuration : nuget_conf_data)
+
+run_target('nuget_pack',
+    command: ['nuget', 'pack', '-OutputDirectory', meson.current_build_dir(),
+              join_paths(meson.current_build_dir(), 'efl_mono.dll.nuspec')]
+)
+
 extra_cs_args = runtime_assemblies
 
 if get_option('mono-beta')