Remove "freebsd" from supported platforms.

DTrace is not yet built by default on FreeBSD releases, so remove
it here, for now.

This also attempts to simplify the GYP targets, by using a dummy
target in the case of a non-DTrace platform.

binding.gyp

@@ -1,9 +1,13 @@
 {
-    'targets': [
-        {
-            'target_name': 'DTraceProviderBindings',
-            'conditions': [
-                ['OS=="mac" or OS=="solaris" or OS=="freebsd"', {
+    'conditions': [
+        ['OS=="mac" or OS=="solaris"', {
+
+            # If we are on the Mac, or a Solaris derivative, attempt
+            # to build the DTrace provider extension.
+
+            'targets': [
+                {
+                    'target_name': 'DTraceProviderBindings',
                     'sources': [
                         'dtrace_provider.cc',
                         'dtrace_probe.cc',
@@ -18,14 +22,10 @@
                     'libraries': [
                         '-L<(module_root_dir)/libusdt -l usdt'
                     ]
-                }]
-            ]
-        },
-        {
-            'target_name': 'libusdt',
-            'type': 'none',
-            'conditions': [
-                ['OS=="mac" or OS=="solaris" or OS=="freebsd"', {
+                },
+                {
+                    'target_name': 'libusdt',
+                    'type': 'none',
                     'actions': [{
                         'inputs': [''],
                         'outputs': [''],
@@ -33,9 +33,22 @@
 	      	        'action': [
                             'sh', 'libusdt-build.sh'
 		        ]
-	            }]
-                }]
+                    }]
+                }
             ]
-        }
+        },
+
+        # If we are not on the Mac or Solaris, DTrace is unavailable. 
+        # This target is necessary because GYP requires at least one
+        # target to exist.
+
+        {
+            'targets': [ 
+                {
+                    'target_name': 'DTraceProviderStub',
+                    'type': 'none'
+                }
+            ]
+        }]
     ]
 }