Expose gdb-index as configure argument

We report it in the overview, but left no way of controlling it

Change-Id: I1d31f2e31bb32566f47069c3776e41033ffb1891
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>

config_help.txt

@@ -95,6 +95,8 @@ Build options:
   -optimized-tools ..... Build optimized host tools even in debug build [no]
   -force-debug-info .... Create symbol files for release builds [no]
   -separate-debug-info . Split off debug information to separate files [no]
+  -gdb-index ........... Index the debug info to speed up GDB
+                         [no; auto if -developer-build with debug info]
   -strip ............... Strip release binaries of unneeded symbols [yes]
   -force-asserts ....... Enable Q_ASSERT even in release builds [no]
   -developer-build ..... Compile and link Qt for developing Qt itself

configure.json

@@ -78,6 +78,7 @@
             "force-debug-info": { "type": "boolean", "name": "force_debug_info" },
             "force-pkg-config": { "type": "void", "name": "pkg-config" },
             "framework": "boolean",
+            "gdb-index": { "type": "boolean", "name": "gdb_index" },
             "gcc-sysroot": "boolean",
             "gcov": "boolean",
             "gnumake": { "type": "boolean", "name": "GNUmake" },