[build-script.py] Provide an option to pass a custom path to the gyb tool

Author: akyrtzi

build-script.py

@@ -507,6 +507,10 @@ def main():
       Path to the FileCheck executable that was built as part of the LLVM
       repository. If not specified, it will be looked up from PATH.
       ''')
+    testing_group.add_argument('--gyb-exec', default=None, help='''
+      Path to the gyb tool. If not specified, it will be looked up relative to
+      the SwiftSyntax directory ('../swift/utils/gyb').
+      ''')
     testing_group.add_argument('--verify-generated-files', action='store_true',
                                help='''
       Instead of generating files using gyb, verify that the files which
@@ -515,6 +519,10 @@ def main():
 
     args = parser.parse_args(sys.argv[1:])
 
+    global GYB_EXEC
+    if args.gyb_exec:
+        GYB_EXEC = args.gyb_exec
+
     if args.install:
         if not args.dylib_dir:
             fatal_error('Must specify directory to install')