[PATCH] Fix warning on gdb/compile/compile.c (C++-ify "triplet_rx")

[Sergio Durigan Junior]

This fixes a GCC warning that happens when compiling
gdb/compile/compile.c on some GCC versions (e.g., "gcc (GCC) 7.2.1
20180104 (Red Hat 7.2.1-6)").

It's a simple patch that converts "triplet_rx" from "char *" to
"std::string", thus guaranteeing that it will be always initialized.

I've regtested this patch and did not find any regressions.  OK to
apply on both master and 8.1 (after creating a bug for it)?

gdb/ChangeLog:
2018-01-16  Sergio Durigan Junior  <sergiodj@redhat.com>

	* compile/compile.c (compile_to_object): Convert "triplet_rx"
	to "std::string".
---
 gdb/compile/compile.c | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/gdb/compile/compile.c b/gdb/compile/compile.c
index 2ee75930ac..47646169c8 100644
--- a/gdb/compile/compile.c
+++ b/gdb/compile/compile.c
@@ -463,7 +463,7 @@ compile_to_object (struct command_line *cmd, const char *cmd_string,
   char **argv;
   int ok;
   struct gdbarch *gdbarch = get_current_arch ();
-  char *triplet_rx;
+  std::string triplet_rx;
   char *error_message;
 
   if (!target_has_execution)
@@ -527,15 +527,15 @@ compile_to_object (struct command_line *cmd, const char *cmd_string,
     }
   else
     {
-      const char *os_rx = osabi_triplet_regexp (gdbarch_osabi (gdbarch));
-      const char *arch_rx = gdbarch_gnu_triplet_regexp (gdbarch);
+      std::string os_rx = osabi_triplet_regexp (gdbarch_osabi (gdbarch));
+      std::string arch_rx = gdbarch_gnu_triplet_regexp (gdbarch);
 
       /* Allow triplets with or without vendor set.  */
-      triplet_rx = concat (arch_rx, "(-[^-]*)?-", os_rx, (char *) NULL);
-      make_cleanup (xfree, triplet_rx);
+      triplet_rx = arch_rx + "(-[^-]*)?-" + os_rx;
 
       if (compiler->fe->ops->version >= GCC_FE_VERSION_1)
-	compiler->fe->ops->set_triplet_regexp (compiler->fe, triplet_rx);
+	compiler->fe->ops->set_triplet_regexp (compiler->fe,
+					       triplet_rx.c_str ());
     }
 
   /* Set compiler command-line arguments.  */
@@ -545,7 +545,8 @@ compile_to_object (struct command_line *cmd, const char *cmd_string,
   if (compiler->fe->ops->version >= GCC_FE_VERSION_1)
     error_message = compiler->fe->ops->set_arguments (compiler->fe, argc, argv);
   else
-    error_message = compiler->fe->ops->set_arguments_v0 (compiler->fe, triplet_rx,
+    error_message = compiler->fe->ops->set_arguments_v0 (compiler->fe,
+							 triplet_rx.c_str (),
 							 argc, argv);
   if (error_message != NULL)
     {
-- 
2.14.3