LIBGAMBIT: Fix building on iOS on current XCode

Backport of gambit/commit/a0dbbcb by @feeley

Author: mgorges

libraries/libgambit/ios.patch

@@ -0,0 +1,49 @@
+--- lib/os_shell.c
++++ lib/os_shell.c
+@@ -1,6 +1,6 @@
+ /* File: "os_shell.c" */
+ 
+-/* Copyright (c) 1994-2017 by Marc Feeley, All Rights Reserved. */
++/* Copyright (c) 1994-2019 by Marc Feeley, All Rights Reserved. */
+ 
+ /*
+  * This module implements the operating system specific routines
+@@ -808,6 +808,15 @@ ___SCMOBJ ___os_environ ___PVOID
+ /* Shell command. */
+ 
+ 
++#define ___USE_system
++
++#ifdef TARGET_OS_IPHONE
++#if TARGET_OS_IPHONE == 1
++#undef ___USE_system
++#endif
++#endif
++
++
+ #if 1
+ 
+ ___SCMOBJ ___os_shell_command
+@@ -816,6 +825,9 @@ ___SCMOBJ ___os_shell_command
+ ___SCMOBJ cmd;)
+ {
+   ___SCMOBJ e;
++
++#ifdef ___USE_system
++
+   char *ccmd;
+ 
+   if ((e = ___SCMOBJ_to_NONNULLCHARSTRING
+@@ -842,6 +854,12 @@ ___SCMOBJ cmd;)
+       ___release_string (ccmd);
+     }
+ 
++#else
++
++  e = ___UNIMPL_ERR;
++
++#endif
++
+   return e;
+ }
+