Add MONO_PATH when running C# executables under Linux or MacOS

[steffen-kiess]

C# executables do not have an rpath. Therefore, when building custom
targets or running tests, MONO_PATH has to be set on Linux or MacOS to
make sure that libraries are found. Under windows this is achieved by
setting PATH (which is used for binaries, native libraries and C#
libraries).

[lgtm-com]

This pull request introduces 1 alert when merging 153261c into 50cf8bc - view on LGTM.com

new alerts:

• 1 for Unused local variable

[steffen-kiess]

I've pushed an updated version which should also work on Windows. Note that on Windows it sets the DEVPATH environment variable and will only work with an application config file like this:

<configuration>
  <runtime>
    <developmentMode developerInstallation="true"/>
  </runtime>
</configuration>

(See https://docs.microsoft.com/en-us/dotnet/framework/configure-apps/file-schema/runtime/developmentmode-element.)

I'm not sure whether there is any better alternative on windows. Other possibilities would be to automatically create a application config with either:

<configuration>  
   <runtime>  
      <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">  
         <probing privatePath="some/path"/>  
      </assemblyBinding>  
   </runtime>  
</configuration>  

or:

<configuration>
  <runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      <dependentAssembly>
        <assemblyIdentity name="libb" />
        <codeBase href="some/path/libb.dll" />
      </dependentAssembly>
    </assemblyBinding>
  </runtime>
</configuration>

but both solutions will only work if the libraries are in some subdirectory of the directory the application is in.

DEVPATH works even if the application is in some other directory, but requires developmentMode to be set. When developmentMode is set but DEVPATH is not set, the application does not seem to start at all (terminates without any error message).