Bug 1259354 - Avoid "checking yasm version" being written out when yasm was not found. r=ted

The reason the "checking" string always appears is that @depends
functions are always called, regardless of the value of the dependency.

This introduces a new decorator @depends_true, which works like
@depends, but the decorated function is not called unless one of the
dependency value resolves to True.

The new decorator can also be used to replace many cases where we do
@depends(foo)
def bar(foo):
    if foo:
        ...

build/moz.configure/toolchain.configure

@@ -8,19 +8,18 @@
 # ==============================================================
 yasm = check_prog('YASM', ['yasm'], allow_missing=True)
 
-@depends(yasm)
+@depends_if(yasm)
 @checking('yasm version')
 @advanced
 def yasm_version(yasm):
-    if yasm:
-        import subprocess
-        try:
-            version = Version(subprocess.check_output(
-                [yasm, '--version']
-            ).splitlines()[0].split()[1])
-            return version
-        except subprocess.CalledProcessError as e:
-            error('Failed to get yasm version: %s' % e.message)
+    import subprocess
+    try:
+        version = Version(subprocess.check_output(
+            [yasm, '--version']
+        ).splitlines()[0].split()[1])
+        return version
+    except subprocess.CalledProcessError as e:
+        error('Failed to get yasm version: %s' % e.message)
 
 # Until we move all the yasm consumers out of old-configure.
 # bug 1257904

build/moz.configure/util.configure

@@ -130,3 +130,16 @@ def delayed_getattr(func, key):
             # In that case, just return None.
             return None
     return result
+
+
+# Like @depends, but the decorated function is only called if one of the
+# arguments it would be called with has a positive value (bool(value) is True)
+@template
+def depends_if(*args):
+    def decorator(func):
+        @depends(*args)
+        def wrapper(*args):
+            if any(arg for arg in args):
+                return func(*args)
+        return wrapper
+    return decorator