meson: try harder to infer build and source directories if called without arguments

If there's no meson.build file in the current directory
and there is one in the parent directory, assume that
we are in the build directory.

mesonbuild/mesonmain.py

@@ -223,10 +223,17 @@ def run(mainfile, args):
         return 0
     args = options.directories
     if len(args) == 0 or len(args) > 2:
+        # if there's a meson.build in the dir above, and not in the current
+        # directory, assume we're in the build directory
+        if len(args) == 0 and not os.path.exists('meson.build') and os.path.exists('../meson.build'):
+            dir1 = '..'
+            dir2 = '.'
+        else:
             print('{} <source directory> <build directory>'.format(sys.argv[0]))
             print('If you omit either directory, the current directory is substituted.')
             print('Run {} --help for more information.'.format(sys.argv[0]))
             return 1
+    else:
         dir1 = args[0]
         if len(args) > 1:
             dir2 = args[1]