Support building with msbuild

[jonpryor]

MSBuild is open source -- has been for ages -- and we'd really like
to migrate all things Mono to MSBuild and deprecate/remove xbuild.

Which means we need to ensure that all of our products build with
msbuild. Which unfortunately isn't the case in Java.Interop:

MSB3375: The file "../../bin/Release/Xamarin.Android.Cecil.dll" does not exist.
...
Java.Interop/MarshalMemberBuilder.cs(9,20): error CS0234:
The type or namespace name `Expressions' does not exist in the namespace `Java.Interop'. Are you missing an assembly reference?
...
    36 Warning(s)
    69 Error(s)

For starters, bring Java.Interop into aligntment/convention with the
xamarin-android repo, so that $(MSBUILD) is the Make variable
to specify the MSBuild engine to use, not $(XBUILD), and allow
$(V) to set $MONO_OPTIONS so that line numbers are included in
stack traces from mono.

Which brings us to the build errors. The MSB3375 error happens when
building e.g. Xamarin.Android.Cecil.csproj from another directory:

$ msbuild src/Xamarin.Android.Cecil/Xamarin.Android.Cecil.csproj
...
.../src/Xamarin.Android.Cecil/Xamarin.Android.Cecil.targets(33,5):
error MSB3375: The file "../../bin/Debug/Xamarin.Android.Cecil.dll" does not exist.
...

The cause, in turn, is because the nested <MSBuild/> invocation
overrides $(OutputPath) when building
external/Cecil/Mono.Cecil.csproj to be
Xamarin.Android.Cecil.csproj's $(OutputPath):

<PropertyGroup>
  <CecilOutputPath>$([System.IO.Path]::GetFullPath ('$(OutputPath)'))</CecilOutputPath>
</PropertyGroup>
<MSBuild
    Properties="OutputPath=$(CecilOutputPath)"
    ...
/>

Or rather, it tries to. It uses Path.GetFullPath() on
$(OutputPath) so that Mono.Cecil.csproj writes the assembly into
the expected Xamarin.Android.Cecil.csproj-specified directory. The
problem is that the Path.GetFullPath() call is relative to the
current working directory, so when:

1. The current working directory isn't the same directory as the
   directory containing Xamarin.Android.Cecil.csproj, and
2. We're building Xamarin.Android.Cecil.csproj, which defines
   $(OutputPath) as e.g. ..\..\Debug

(1) and (2) interplay so that $(CecilOutputPath) becomes e.g.
$CWD/../../bin/Debug, which is not the correct directory, and may
(will!) be outside of the repo checkout directory.

Modify the $(CecilOutputPath) definition so that it instead assumes
that $(OutputPath) is a directory relative to
$(MSBuildThisFileDirectory), equivalent to C#:

var OutputPath                = ...
var MSBuildThisFileDirectory  = ...

var CecilOutputPath = Path.Combine (MSBuildThisFileDirectory, OutputPath);
CecilOutputPath     = Path.GetFullPath (CecilOutputPath);

This fixes the above interplay by "inserting" use of
$(MSBuildThisFileDirectory) into the "normal"
msbuild Xamarin.Android.Cecil.csproj invocation path, ensuring that
output paths behave as expected.

This in turn allows Xamarin.Android.Cecil.dll to be placed into the
correct directory, which in turn fixes all of the other C# errors
(present apparently because Xamarin.Android.Cecil.dll couldn't be
resolved).