Add autoconf check for W^X platforms that require a trampoline table.

This adds the FFI_EXEC_TRAMPOLINE_TABLE. The flag is enabled for
arm-apple-darwin, where PROT_EXEC on writable (or formerly writable) pages is
not permitted for sandboxed binaries.

configure.ac

@@ -316,6 +316,11 @@ if test x$TARGET = xX86 || test x$TARGET = xX86_WIN32 || test x$TARGET = xX86_64
 fi
 
 case "$target" in
+     *arm*-apple-darwin*)
+       AC_DEFINE(FFI_EXEC_TRAMPOLINE_TABLE, 1,
+                 [Cannot use PROT_EXEC on this target, so, we revert to
+                   alternative means])
+     ;;
      *-apple-darwin10* | *-*-freebsd* | *-*-openbsd* | *-pc-solaris*)
        AC_DEFINE(FFI_MMAP_EXEC_WRIT, 1,
                  [Cannot use malloc on this target, so, we revert to

fficonfig.h.in

@@ -17,6 +17,9 @@
 /* Define this if you want extra debugging. */
 #undef FFI_DEBUG
 
+/* Cannot use PROT_EXEC on this target, so, we revert to alternative means */
+#undef FFI_EXEC_TRAMPOLINE_TABLE
+
 /* Cannot use malloc on this target, so, we revert to alternative means */
 #undef FFI_MMAP_EXEC_WRIT