add "--no-generate-GDEF" command line parameter

This adds the "--no-generate-GDEF" command line parameter which sets
the "generate_GDEF" variable which in turn is passed over to
glyhpsLib invocations to determine whether to auto-generate a GDEF
table or not.

Fixes googlefonts#702.

Lib/fontmake/__main__.py

@@ -224,6 +224,13 @@ def main(args=None):
         "interpolating. Use if you share feature files of masters in "
         "external files, as instances can end up elsewhere.",
     )
+    outputGroup.add_argument(
+        "--no-generate-GDEF",
+        dest="generate_GDEF",
+        action="store_false",
+        help="Do not auto-generate a GDEF table, but keep an existing one "
+        "intact.",
+    )
 
     contourGroup = parser.add_argument_group(title="Handling of contours")
     contourGroup.add_argument(

Lib/fontmake/font_project.py

@@ -137,6 +137,7 @@ def build_master_ufos(
         family_name=None,
         mti_source=None,
         write_skipexportglyphs=True,
+        generate_GDEF=True,
     ):
         """Build UFOs and MutatorMath designspace from Glyphs source."""
         import glyphsLib
@@ -170,6 +171,7 @@ def build_master_ufos(
             instance_dir=instance_dir,
             write_skipexportglyphs=write_skipexportglyphs,
             ufo_module=ufoLib2,
+            generate_GDEF=generate_GDEF,
         )
 
         masters = {}
@@ -393,6 +395,7 @@ def save_otfs(
         inplace=True,
         cff_version=1,
         subroutinizer=None,
+        generate_GDEF=True,
     ):
         """Build OpenType binaries from UFOs.
 
@@ -646,6 +649,7 @@ def run_from_glyphs(
         family_name=None,
         mti_source=None,
         write_skipexportglyphs=True,
+        generate_GDEF=True,
         **kwargs,
     ):
         """Run toolchain from Glyphs source.
@@ -675,6 +679,7 @@ def run_from_glyphs(
             family_name=family_name,
             mti_source=mti_source,
             write_skipexportglyphs=write_skipexportglyphs,
+            generate_GDEF=generate_GDEF,
         )
         try:
             self.run_from_designspace(designspace_path, **kwargs)