[xabuild] Multiple Symlinking xabuilds (dotnet#940)

This commit fixes a couple of issues which appeared when
using `xabuild.exe` on macOS. Firstly if the symlink exists
already the SymbolicLink code will error out and fail the
build completely. This happens because if you run two or
more `xabuild.exe` instances at the same time they trip over
each other trying to create the symlink.

So rather than error'ing completely we should only error if
the link does NOT exist. If it does exist after an attempted
creation we should ignore the exception.

The other is about where we look for extensions. Xamarin.Android
is installed on macOS into

	/Library/Frameworks/Mono.framework/External/xbuild

this was not included in the search path for MSBuild, so it never
manages to find the required `.targets` files.

tools/xabuild/SymbolicLink.cs

@@ -16,15 +16,21 @@ public static bool Create (string source, string target)
 					if (!CreateSymbolicLink (source, target, SymbolLinkFlag.Directory | SymbolLinkFlag.AllowUnprivilegedCreate) &&
 						!CreateSymbolicLink (source, target, SymbolLinkFlag.Directory)) {
 						var error = new Win32Exception ().Message;
-						Console.Error.WriteLine ($"Unable to create symbolic link from `{source}` to `{target}`: {error}");
-						return false;
+						var result = Directory.Exists (source);
+						if (!result)
+							Console.Error.WriteLine ($"Unable to create symbolic link from `{source}` to `{target}`: {error}");
+						return result;
 					}
 				} else {
 					try {
+						var sourceInfo = new UnixFileInfo (source);
 						var fileInfo = new UnixFileInfo (target);
 						fileInfo.CreateSymbolicLink (source);
-					} catch (Exception exc) {
-						Console.Error.WriteLine ($"Unable to create symbolic link from `{source}` to `{target}`: {exc.Message}");
+					} catch (UnixIOException exc) {
+						if (exc.ErrorCode == Mono.Unix.Native.Errno.EEXIST) {
+							return true;
+						}
+						Console.Error.WriteLine ($"Unable to create symbolic link from `{source}` to `{target}`: {exc}");
 						return false;
 					}
 				}

tools/xabuild/XABuildPaths.cs

@@ -114,10 +114,11 @@ public XABuildPaths ()
 				SearchPathsOS            = "windows";
 			} else {
 				string mono              = IsMacOS ? "/Library/Frameworks/Mono.framework/Versions/Current/lib/mono" : "/usr/lib/mono";
+				string monoExternal      = IsMacOS ? "/Library/Frameworks/Mono.framework/External/" : "/usr/lib/mono";
 				MSBuildPath              = Path.Combine (mono, "msbuild");
 				MSBuildBin               = Path.Combine (MSBuildPath, "15.0", "bin");
 				MSBuildConfig            = Path.Combine (MSBuildBin, "MSBuild.dll.config");
-				ProjectImportSearchPaths = new [] { MSBuildPath, Path.Combine (mono, "xbuild") };
+				ProjectImportSearchPaths = new [] { MSBuildPath, Path.Combine (mono, "xbuild"), Path.Combine (monoExternal, "xbuild") };
 				SystemProfiles           = Path.Combine (mono, "xbuild-frameworks");
 				XABuildConfig            = Path.Combine (XABuildDirectory, "MSBuild.dll.config");
 				SearchPathsOS            = IsMacOS ? "osx" : "unix";