gnu: gdb: Fix build with Python >= 3.7.

* gnu/packages/patches/gdb-python-3.7.patch: New file.
* gnu/local.mk (dist_patch_DATA): Register it.
* gnu/packages/gdb.scm (gdb)[source](patches): New field.

gnu/local.mk

@@ -709,6 +709,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/gd-CVE-2018-5711.patch			\
   %D%/packages/patches/gd-fix-tests-on-i686.patch		\
   %D%/packages/patches/gd-freetype-test-failure.patch		\
+  %D%/packages/patches/gdb-python-3.7.patch			\
   %D%/packages/patches/gemma-intel-compat.patch			\
   %D%/packages/patches/geoclue-config.patch			\
   %D%/packages/patches/ghc-8.0-fall-back-to-madv_dontneed.patch \

gnu/packages/gdb.scm

@@ -42,6 +42,7 @@ (define-public gdb
              (method url-fetch)
              (uri (string-append "mirror://gnu/gdb/gdb-"
                                  version ".tar.xz"))
+             (patches (search-patches "gdb-python-3.7.patch"))
              (sha256
               (base32
                "0d2bpqk58fqlx21rbnk8mbcjlggzc9kb5sjirrfrrrjq70ka0qdg"))))

gnu/packages/patches/gdb-python-3.7.patch

@@ -0,0 +1,52 @@
+Fix build failure with Python 3.7 and newer.
+
+Taken from this upstream commit, sans ChangeLog update:
+
+https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commitdiff;h=aeab512851bf6ed623d1c6c4305b6ce05e51a10c
+
+diff --git a/gdb/python/python.c b/gdb/python/python.c
+index 1805c90..20fc674 100644
+--- a/gdb/python/python.c
++++ b/gdb/python/python.c
+@@ -1667,6 +1667,17 @@ finalize_python (void *ignore)
+   restore_active_ext_lang (previous_active);
+ }
+ 
++#ifdef IS_PY3K
++/* This is called via the PyImport_AppendInittab mechanism called
++   during initialization, to make the built-in _gdb module known to
++   Python.  */
++PyMODINIT_FUNC
++init__gdb_module (void)
++{
++  return PyModule_Create (&python_GdbModuleDef);
++}
++#endif
++
+ static bool
+ do_start_initialization ()
+ {
+@@ -1707,6 +1718,9 @@ do_start_initialization ()
+      remain alive for the duration of the program's execution, so
+      it is not freed after this call.  */
+   Py_SetProgramName (progname_copy);
++
++  /* Define _gdb as a built-in module.  */
++  PyImport_AppendInittab ("_gdb", init__gdb_module);
+ #else
+   Py_SetProgramName (progname.release ());
+ #endif
+@@ -1716,9 +1730,7 @@ do_start_initialization ()
+   PyEval_InitThreads ();
+ 
+ #ifdef IS_PY3K
+-  gdb_module = PyModule_Create (&python_GdbModuleDef);
+-  /* Add _gdb module to the list of known built-in modules.  */
+-  _PyImport_FixupBuiltin (gdb_module, "_gdb");
++  gdb_module = PyImport_ImportModule ("_gdb");
+ #else
+   gdb_module = Py_InitModule ("_gdb", python_GdbMethods);
+ #endif
+-- 
+2.9.3
+