Updated define constants to address bugzilla 24797 and add missing supported features.

*   Removed FX_NO_STRUCTURAL_EQUALITY, FX_NO_CUSTOMATTRIBUTEDATA as these
are both provided in mscorlib for monotouch/monodroid.
*(FX_NO_STRUCTURAL_EQUALITY actually causes an issue with array
comparisons within tuples, see [bugzilla 24797](https://bugzilla.xamarin.com/show_bug.cgi?id=24797))*

*   Removed FX_NO_BIGINT_CULTURE_PARSE as its already covered by
FX_NO_BIGINT

*   Added FX_ATLEAST_45, FSHARP_CORE_4_5 as .Net 4.5 features defined
within are supported by monotouch/monodroid
MonoDroid FSharp.Core fails to build without it, the mscorlib must be different.  There's also an explicit mscorlib referenced in MonoTouch unlike android. 

I don't know who put together the android one so I can't comment on the reasons behind it.  



> On 27 Nov 2014, at 19:26, Don Syme <notifications@github.com> wrote:
> 
> In src/FSharpSource.targets:
> 
> > @@ -141,9 +141,9 @@ Some other NuGET monikers to support in the future, see http://docs.nuget.org/do
> >    <PropertyGroup Condition="'$(TargetFramework)'=='monodroid'">
> >      <TargetFrameworkVersion>v2.1</TargetFrameworkVersion>
> >      <TargetFrameworkOutputDirectory>$(TargetFramework)</TargetFrameworkOutputDirectory>
> > -    <DefineConstants>$(DefineConstants);FX_NO_STRUCTURAL_EQUALITY</DefineConstants>
> >      <DefineConstants>$(DefineConstants);FX_NO_CUSTOMATTRIBUTEDATA</DefineConstants>
> Why is FX_NO_CUSTOMATTRIBUTEDATA still here? thanks
> 
> —
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