[xbuild] Use the env var $MSBuildExtensionsPath before trying other p…

…aths.

Change the order of the "search paths", so that the environment
variable can always override it.
Update the man page to reflect the change.

man/xbuild.1

@@ -73,8 +73,8 @@ to $prefix/lib/mono/xbuild . When used in Import, like:
 
 xbuild tries various values for the msbuild property $(MSBuildExtensionsPath), in order:
 
-  1. /Library/Frameworks/Mono.framework/External/xbuild on Mac OSX.
-  2. Paths specified in the environment variable $MSBuildExtensionsPath.
+  1. Paths specified in the environment variable $MSBuildExtensionsPath.
+  2. /Library/Frameworks/Mono.framework/External/xbuild on Mac OSX.
   3. $XDG_CONFIG_HOME/xbuild/tasks (or Environment.SpecialFolder.ApplicationData)
   4. $prefix/lib/mono/xbuild (default location)
 

mcs/class/Microsoft.Build.Engine/Microsoft.Build.BuildEngine/Import.cs

@@ -120,10 +120,10 @@ bool EvaluateAsMSBuildExtensionsPath (string file, string property_name, out str
 
 			string envvar = Environment.GetEnvironmentVariable (property_name);
 			envvar = String.Join (":", new string [] {
+						(envvar ?? String.Empty),
 						// For mac osx, look in the 'External' dir on macosx,
 						// see bug #663180
 						MSBuildUtils.RunningOnMac ? MacOSXExternalXBuildDir : String.Empty,
-						(envvar ?? String.Empty),
 						DotConfigExtensionsPath});
 
 			string [] paths = envvar.Split (new char [] {':'}, StringSplitOptions.RemoveEmptyEntries);